[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Recent toolchains default to using the hardware feature for unaligned access on
ARM v7, rather than doing the software fallback. According to ARM this is safe
as all v7 implementations have to support this feature.
(http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)

To avoid CPU hangs when doing unaligned memory access, we have to turn off
alignment checking in our CPU initialisation code.
(http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)

Signed-off-by: Lucas Stach <dev@lynxeye.de>
CC: Albert ARIBAUD <albert.u.boot@aribaud.net>
---
 arch/arm/cpu/armv7/start.S |    2 +-
 1 Datei ge?ndert, 1 Zeile hinzugef?gt(+), 1 Zeile entfernt(-)

diff --git a/arch/arm/cpu/armv7/start.S b/arch/arm/cpu/armv7/start.S
index 261835b..52f7f6e 100644
--- a/arch/arm/cpu/armv7/start.S
+++ b/arch/arm/cpu/armv7/start.S
@@ -316,7 +316,7 @@ ENTRY(cpu_init_cp15)
 	mrc	p15, 0, r0, c1, c0, 0
 	bic	r0, r0, #0x00002000	@ clear bits 13 (--V-)
 	bic	r0, r0, #0x00000007	@ clear bits 2:0 (-CAM)
-	orr	r0, r0, #0x00000002	@ set bit 1 (--A-) Align
+	bic	r0, r0, #0x00000002	@ clear bit 1 (--A-) Align
 	orr	r0, r0, #0x00000800	@ set bit 11 (Z---) BTB
 #ifdef CONFIG_SYS_ICACHE_OFF
 	bic	r0, r0, #0x00001000	@ clear bit 12 (I) I-cache
-- 
1.7.10.2

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Stephen Warren]

On 06/05/2012 11:47 AM, Lucas Stach wrote:
> Recent toolchains default to using the hardware feature for unaligned access on
> ARM v7, rather than doing the software fallback. According to ARM this is safe
> as all v7 implementations have to support this feature.
> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
> 
> To avoid CPU hangs when doing unaligned memory access, we have to turn off
> alignment checking in our CPU initialisation code.
> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)

Does this behavior change trickle down to Linux/... too, or would an OS
completely re-initialize this state, and hence not be affected?

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Hi Stephen,

Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
> On 06/05/2012 11:47 AM, Lucas Stach wrote:
> > Recent toolchains default to using the hardware feature for unaligned access on
> > ARM v7, rather than doing the software fallback. According to ARM this is safe
> > as all v7 implementations have to support this feature.
> > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
> > 
> > To avoid CPU hangs when doing unaligned memory access, we have to turn off
> > alignment checking in our CPU initialisation code.
> > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
> 
> Does this behavior change trickle down to Linux/... too, or would an OS
> completely re-initialize this state, and hence not be affected?
> 

Linux in particular does reinitialize this state and I expect any
reasonable OS to do so.

-- Lucas

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Albert ARIBAUD]

Hi Lucas,

On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach <dev@lynxeye.de> wrote:
> Hi Stephen,
> 
> Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
> > On 06/05/2012 11:47 AM, Lucas Stach wrote:
> > > Recent toolchains default to using the hardware feature for
> > > unaligned access on ARM v7, rather than doing the software
> > > fallback. According to ARM this is safe as all v7 implementations
> > > have to support this feature.
> > > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
> > > 
> > > To avoid CPU hangs when doing unaligned memory access, we have to
> > > turn off alignment checking in our CPU initialisation code.
> > > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
> > 
> > Does this behavior change trickle down to Linux/... too, or would
> > an OS completely re-initialize this state, and hence not be
> > affected?
> > 
> 
> Linux in particular does reinitialize this state and I expect any
> reasonable OS to do so.

Then what is the point of enabling it on U-Boot? Does it fix some issue
whereby some mis-aligned piece of data cannot be properly aligned?

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Am Freitag, den 22.06.2012, 11:15 +0200 schrieb Albert ARIBAUD:
> Hi Lucas,
> 
> On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach <dev@lynxeye.de> wrote:
> > Hi Stephen,
> > 
> > Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
> > > On 06/05/2012 11:47 AM, Lucas Stach wrote:
> > > > Recent toolchains default to using the hardware feature for
> > > > unaligned access on ARM v7, rather than doing the software
> > > > fallback. According to ARM this is safe as all v7 implementations
> > > > have to support this feature.
> > > > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
> > > > 
> > > > To avoid CPU hangs when doing unaligned memory access, we have to
> > > > turn off alignment checking in our CPU initialisation code.
> > > > (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
> > > 
> > > Does this behavior change trickle down to Linux/... too, or would
> > > an OS completely re-initialize this state, and hence not be
> > > affected?
> > > 
> > 
> > Linux in particular does reinitialize this state and I expect any
> > reasonable OS to do so.
> 
> Then what is the point of enabling it on U-Boot? Does it fix some issue
> whereby some mis-aligned piece of data cannot be properly aligned?
> 
Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain.
Fixing the alignment of some of the structures in the USB code should
also be done, but this is a whole lot more invasive and requires some
more thought, as the discussion about this on LKML shows. The issue
doesn't show for older toolchains, as they by default emit code to work
around unaligned accesses.

This patch fixes all unaligned issues, that may appear with recent
toolchains. We avoid having to instruct the toolchain to work around
unaligned accesses and gain better performance in cases where it is
needed.

Thanks,
Lucas

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Albert ARIBAUD]

Hi Lucas,

> > > Linux in particular does reinitialize this state and I expect any
> > > reasonable OS to do so.
> > 
> > Then what is the point of enabling it on U-Boot? Does it fix some
> > issue whereby some mis-aligned piece of data cannot be properly
> > aligned?
> > 
> Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain.
> Fixing the alignment of some of the structures in the USB code should
> also be done, but this is a whole lot more invasive and requires some
> more thought, as the discussion about this on LKML shows. The issue
> doesn't show for older toolchains, as they by default emit code to
> work around unaligned accesses.
> 
> This patch fixes all unaligned issues, that may appear with recent
> toolchains. We avoid having to instruct the toolchain to work around
> unaligned accesses and gain better performance in cases where it is
> needed.

I am not too happy with enabling a lax behavior only to avoid an
issue which apparently is diagnosed and could / should be fixed at
its root. Can you point me to the relevant LKML thread
so that I get the details and understand what prevents fixing this by
'simply' aligning the USB structures?

> Thanks,
> Lucas

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Hi Albert,

Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
> Hi Lucas,
> 
> > > > Linux in particular does reinitialize this state and I expect any
> > > > reasonable OS to do so.
> > > 
> > > Then what is the point of enabling it on U-Boot? Does it fix some
> > > issue whereby some mis-aligned piece of data cannot be properly
> > > aligned?
> > > 
> > Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain.
> > Fixing the alignment of some of the structures in the USB code should
> > also be done, but this is a whole lot more invasive and requires some
> > more thought, as the discussion about this on LKML shows. The issue
> > doesn't show for older toolchains, as they by default emit code to
> > work around unaligned accesses.
> > 
> > This patch fixes all unaligned issues, that may appear with recent
> > toolchains. We avoid having to instruct the toolchain to work around
> > unaligned accesses and gain better performance in cases where it is
> > needed.
> 
> I am not too happy with enabling a lax behavior only to avoid an
> issue which apparently is diagnosed and could / should be fixed at
> its root. Can you point me to the relevant LKML thread
> so that I get the details and understand what prevents fixing this by
> 'simply' aligning the USB structures?

I'm with you that the issue for this particular fault that I ran into
should be fixed at it's root and I will do so as soon as I have enough
time to do so, i.e. within the next three weeks.
You can find a thread about this here:
https://lkml.org/lkml/2011/4/27/278
The problem here is that simply removing the packed attribute is not the
right thing to do for structures that are used for accessing hardware
registers. I have to read a bit more of the USB code to come up with the
right thing to do for every structure there.

But apart from this, we certainly have situations where we have
unaligned accesses that are justified and could not be removed.
Activating the aligned access hardware check is crippling a feature of
the ARMv7 architecture that's apparently useful enough that all recent
toolchains default to using it and not using compiler side workarounds.
Furthermore setting the check bit doesn't even deactivate unaligned
access (there is also a bit for this, which is forced to 1 by all v7
implementations), but just traps the processor in case you care about
such unaligned accesses. Linux for example only sets this check bit if
you choose to install a trap handler for this.

I cannot see how enabling a hardware feature can be seen as allowing of
lax behaviour. As some of the USB structs are used to access hardware
registers, we can not align every struct there. Our options are either:
1. instruct the toolchain to insert workaround code or
2. allow v7 hardware to do the unaligned access on it's own
My comment about fixing the USB code applies only to part of the structs
used there as some of them generate _unnecessary_ unaligned accesses,
the issue that all unaligned accesses fail with current U-Boot built
with a recent toolchain has to be fixed either way and is exactly what
this patch does.

Thanks,
Lucas

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Aneesh V]

+Tom

Hi Lucas,

On 06/22/2012 04:47 AM, Lucas Stach wrote:
> Hi Albert,
>
> Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
>> Hi Lucas,
>>
>>>>> Linux in particular does reinitialize this state and I expect any
>>>>> reasonable OS to do so.
>>>>
>>>> Then what is the point of enabling it on U-Boot? Does it fix some
>>>> issue whereby some mis-aligned piece of data cannot be properly
>>>> aligned?
>>>>
>>> Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain.
>>> Fixing the alignment of some of the structures in the USB code should
>>> also be done, but this is a whole lot more invasive and requires some
>>> more thought, as the discussion about this on LKML shows. The issue
>>> doesn't show for older toolchains, as they by default emit code to
>>> work around unaligned accesses.
>>>
>>> This patch fixes all unaligned issues, that may appear with recent
>>> toolchains. We avoid having to instruct the toolchain to work around
>>> unaligned accesses and gain better performance in cases where it is
>>> needed.
>>
>> I am not too happy with enabling a lax behavior only to avoid an
>> issue which apparently is diagnosed and could / should be fixed at
>> its root. Can you point me to the relevant LKML thread
>> so that I get the details and understand what prevents fixing this by
>> 'simply' aligning the USB structures?
>
> I'm with you that the issue for this particular fault that I ran into
> should be fixed at it's root and I will do so as soon as I have enough
> time to do so, i.e. within the next three weeks.
> You can find a thread about this here:
> https://lkml.org/lkml/2011/4/27/278
> The problem here is that simply removing the packed attribute is not the
> right thing to do for structures that are used for accessing hardware
> registers. I have to read a bit more of the USB code to come up with the
> right thing to do for every structure there.
>
> But apart from this, we certainly have situations where we have
> unaligned accesses that are justified and could not be removed.
> Activating the aligned access hardware check is crippling a feature of
> the ARMv7 architecture that's apparently useful enough that all recent
> toolchains default to using it and not using compiler side workarounds.
> Furthermore setting the check bit doesn't even deactivate unaligned
> access (there is also a bit for this, which is forced to 1 by all v7
> implementations), but just traps the processor in case you care about
> such unaligned accesses. Linux for example only sets this check bit if
> you choose to install a trap handler for this.
>
> I cannot see how enabling a hardware feature can be seen as allowing of
> lax behaviour. As some of the USB structs are used to access hardware
> registers, we can not align every struct there. Our options are either:
> 1. instruct the toolchain to insert workaround code or
> 2. allow v7 hardware to do the unaligned access on it's own
> My comment about fixing the USB code applies only to part of the structs
> used there as some of them generate _unnecessary_ unaligned accesses,
> the issue that all unaligned accesses fail with current U-Boot built
> with a recent toolchain has to be fixed either way and is exactly what
> this patch does.

I think this issue was discussed before here(I haven't gone through all
the details of your problem, but it looks similar). And I think Tom
fixed this by wrapping the problematic accesses with get/set_unaligned().

Please look at this thread, especially starting from my post reporting
the OMAP4 regression:
http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/

best regards,
Aneesh

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Aneesh V]

On 06/22/2012 03:11 PM, Aneesh V wrote:
> +Tom
>
> Hi Lucas,
>
> On 06/22/2012 04:47 AM, Lucas Stach wrote:
>> Hi Albert,
>>
>> Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
>>> Hi Lucas,
>>>
>>>>>> Linux in particular does reinitialize this state and I expect any
>>>>>> reasonable OS to do so.
>>>>>
>>>>> Then what is the point of enabling it on U-Boot? Does it fix some
>>>>> issue whereby some mis-aligned piece of data cannot be properly
>>>>> aligned?
>>>>>
>>>> Yes, it fixes U-Boot USB on Tegra, when built with a recent toolchain.
>>>> Fixing the alignment of some of the structures in the USB code should
>>>> also be done, but this is a whole lot more invasive and requires some
>>>> more thought, as the discussion about this on LKML shows. The issue
>>>> doesn't show for older toolchains, as they by default emit code to
>>>> work around unaligned accesses.
>>>>
>>>> This patch fixes all unaligned issues, that may appear with recent
>>>> toolchains. We avoid having to instruct the toolchain to work around
>>>> unaligned accesses and gain better performance in cases where it is
>>>> needed.
>>>
>>> I am not too happy with enabling a lax behavior only to avoid an
>>> issue which apparently is diagnosed and could / should be fixed at
>>> its root. Can you point me to the relevant LKML thread
>>> so that I get the details and understand what prevents fixing this by
>>> 'simply' aligning the USB structures?
>>
>> I'm with you that the issue for this particular fault that I ran into
>> should be fixed at it's root and I will do so as soon as I have enough
>> time to do so, i.e. within the next three weeks.
>> You can find a thread about this here:
>> https://lkml.org/lkml/2011/4/27/278
>> The problem here is that simply removing the packed attribute is not the
>> right thing to do for structures that are used for accessing hardware
>> registers. I have to read a bit more of the USB code to come up with the
>> right thing to do for every structure there.
>>
>> But apart from this, we certainly have situations where we have
>> unaligned accesses that are justified and could not be removed.
>> Activating the aligned access hardware check is crippling a feature of
>> the ARMv7 architecture that's apparently useful enough that all recent
>> toolchains default to using it and not using compiler side workarounds.
>> Furthermore setting the check bit doesn't even deactivate unaligned
>> access (there is also a bit for this, which is forced to 1 by all v7
>> implementations), but just traps the processor in case you care about
>> such unaligned accesses. Linux for example only sets this check bit if
>> you choose to install a trap handler for this.
>>
>> I cannot see how enabling a hardware feature can be seen as allowing of
>> lax behaviour. As some of the USB structs are used to access hardware
>> registers, we can not align every struct there. Our options are either:
>> 1. instruct the toolchain to insert workaround code or
>> 2. allow v7 hardware to do the unaligned access on it's own
>> My comment about fixing the USB code applies only to part of the structs
>> used there as some of them generate _unnecessary_ unaligned accesses,
>> the issue that all unaligned accesses fail with current U-Boot built
>> with a recent toolchain has to be fixed either way and is exactly what
>> this patch does.
>
> I think this issue was discussed before here(I haven't gone through all
> the details of your problem, but it looks similar). And I think Tom
> fixed this by wrapping the problematic accesses with get/set_unaligned().
>
> Please look at this thread, especially starting from my post reporting
> the OMAP4 regression:
> http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/

BTW, I agree that enabling un-aligned access is not a bad idea.

br,
Aneeesh

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Albert ARIBAUD]

Hi Aneesh,

On Fri, 22 Jun 2012 15:13:39 -0700, Aneesh V <aneesh@ti.com> wrote:
> On 06/22/2012 03:11 PM, Aneesh V wrote:
> > +Tom
> >
> > Hi Lucas,
> >
> > On 06/22/2012 04:47 AM, Lucas Stach wrote:
> >> Hi Albert,
> >>
> >> Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:

> >>> I am not too happy with enabling a lax behavior only to avoid an
> >>> issue which apparently is diagnosed and could / should be fixed at
> >>> its root. Can you point me to the relevant LKML thread
> >>> so that I get the details and understand what prevents fixing
> >>> this by 'simply' aligning the USB structures?
> >>
> >> I'm with you that the issue for this particular fault that I ran
> >> into should be fixed at it's root and I will do so as soon as I
> >> have enough time to do so, i.e. within the next three weeks.
> >> You can find a thread about this here:
> >> https://lkml.org/lkml/2011/4/27/278

From what I understand, the issue was not to allow unaligned access at
the hardware level, but to modify the attributes of the structure,
first by removing the packed attribute, then by reinstating the packed
attribute along with align(4).

> >> But apart from this, we certainly have situations where we have
> >> unaligned accesses that are justified and could not be removed.
> >> [...]
> >> I cannot see how enabling a hardware feature can be seen as
> >> allowing of lax behaviour. As some of the USB structs are used to
> >> access hardware registers, we can not align every struct there.

If the access is in true RAM, then we can always realign the data; and
I don't know of memory-mapped registers which would be unaligned wrt
their width. If some USB controller is designed so, then the fix should
only and explicitly affect that controller, because we don't know it it
will always be used with an ARM implementation that can do unaligned
accesses.

> > I think this issue was discussed before here(I haven't gone through
> > all the details of your problem, but it looks similar). And I think
> > Tom fixed this by wrapping the problematic accesses with
> > get/set_unaligned().

Could be a solution if the structures themselves cannot be fixed.

> > Please look at this thread, especially starting from my post
> > reporting the OMAP4 regression:
> > http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/

Thanks for the reference. There seems to have been no confirmation that
the structures involved needed packing in the first place, and my
general opinion on packing structures is "DO NOT" -- if packing a
structure has an effect, it can alway be to de-align some field, which
barely makes sense as far as HW prgramming is concerned (I can only see
some point in packing a struct when you deal with network layer 7 data
in very special cases).

> BTW, I agree that enabling un-aligned access is not a bad idea.

Just being "not a bad idea" is not enough for me to accept this. It will
have to be the sole sound solution to a problem, and at this point, I
do not think it is as far as USB structure mis-alignement issues are
concerned.

> br,
> Aneeesh

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[V, Aneesh]

Hi Albert,

On Sat, Jun 23, 2012 at 2:01 AM, Albert ARIBAUD
<albert.u.boot@aribaud.net> wrote:
> Hi Aneesh,
>
> On Fri, 22 Jun 2012 15:13:39 -0700, Aneesh V <aneesh@ti.com> wrote:
>> On 06/22/2012 03:11 PM, Aneesh V wrote:
>> > +Tom
>> >
>> > Hi Lucas,
>> >
>> > On 06/22/2012 04:47 AM, Lucas Stach wrote:
>> >> Hi Albert,
>> >>
>> >> Am Freitag, den 22.06.2012, 13:16 +0200 schrieb Albert ARIBAUD:
>
>> >>> I am not too happy with enabling a lax behavior only to avoid an
>> >>> issue which apparently is diagnosed and could / should be fixed at
>> >>> its root. Can you point me to the relevant LKML thread
>> >>> so that I get the details and understand what prevents fixing
>> >>> this by 'simply' aligning the USB structures?
>> >>
>> >> I'm with you that the issue for this particular fault that I ran
>> >> into should be fixed at it's root and I will do so as soon as I
>> >> have enough time to do so, i.e. within the next three weeks.
>> >> You can find a thread about this here:
>> >> https://lkml.org/lkml/2011/4/27/278
>
> From what I understand, the issue was not to allow unaligned access at
> the hardware level, but to modify the attributes of the structure,
> first by removing the packed attribute, then by reinstating the packed
> attribute along with align(4).
>
>> >> But apart from this, we certainly have situations where we have
>> >> unaligned accesses that are justified and could not be removed.
>> >> [...]
>> >> I cannot see how enabling a hardware feature can be seen as
>> >> allowing of lax behaviour. As some of the USB structs are used to
>> >> access hardware registers, we can not align every struct there.
>
> If the access is in true RAM, then we can always realign the data; and
> I don't know of memory-mapped registers which would be unaligned wrt
> their width. If some USB controller is designed so, then the fix should
> only and explicitly affect that controller, because we don't know it it
> will always be used with an ARM implementation that can do unaligned
> accesses.
>
>> > I think this issue was discussed before here(I haven't gone through
>> > all the details of your problem, but it looks similar). And I think
>> > Tom fixed this by wrapping the problematic accesses with
>> > get/set_unaligned().
>
> Could be a solution if the structures themselves cannot be fixed.
>
>> > Please look at this thread, especially starting from my post
>> > reporting the OMAP4 regression:
>> > http://thread.gmane.org/gmane.comp.boot-loaders.u-boot/113347/
>
> Thanks for the reference. There seems to have been no confirmation that
> the structures involved needed packing in the first place, and my
> general opinion on packing structures is "DO NOT" -- if packing a
> structure has an effect, it can alway be to de-align some field, which
> barely makes sense as far as HW prgramming is concerned (I can only see
> some point in packing a struct when you deal with network layer 7 data
> in very special cases).
>
>> BTW, I agree that enabling un-aligned access is not a bad idea.
>
> Just being "not a bad idea" is not enough for me to accept this. It will
> have to be the sole sound solution to a problem, and at this point, I
> do not think it is as far as USB structure mis-alignement issues are
> concerned.

My point is that enabling un-aligned accesses in itsown merit
is not a bad idea, not as a solution to this problem. I have seen
it being enabled in HLOS environment. TI's Symbian port for
instance had it enabled for OMAP3. I don't know why Linux too
shoudln't take advantage of such hw features.
Perhaps you don't want to take it at this point of time to force
the real solution to the USB problem, which is reasonable.

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Måns Rullgård]

Albert ARIBAUD <albert.u.boot@aribaud.net> writes:

>> >> I cannot see how enabling a hardware feature can be seen as
>> >> allowing of lax behaviour. As some of the USB structs are used to
>> >> access hardware registers, we can not align every struct there.
>
> If the access is in true RAM, then we can always realign the data; and
> I don't know of memory-mapped registers which would be unaligned wrt
> their width. If some USB controller is designed so, then the fix should
> only and explicitly affect that controller, because we don't know it it
> will always be used with an ARM implementation that can do unaligned
> accesses.

The ARM architecture does not permit unaligned accesses to strongly
ordered or device memory, so MMIO register accesses are always aligned.

-- 
M?ns Rullg?rd
mans at mansr.com

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Hi Albert,

Am Samstag, den 23.06.2012, 11:01 +0200 schrieb Albert ARIBAUD:
[snip]
> > >> But apart from this, we certainly have situations where we have
> > >> unaligned accesses that are justified and could not be removed.
> > >> [...]
> > >> I cannot see how enabling a hardware feature can be seen as
> > >> allowing of lax behaviour. As some of the USB structs are used to
> > >> access hardware registers, we can not align every struct there.
> 
> If the access is in true RAM, then we can always realign the data; and
> I don't know of memory-mapped registers which would be unaligned wrt
> their width. If some USB controller is designed so, then the fix should
> only and explicitly affect that controller, because we don't know it it
> will always be used with an ARM implementation that can do unaligned
> accesses.

My point is: on ARM platforms that can't do unaligned access in hardware
the toolchain will automaticly emit helper code to work around this. On
an ARMv7 platform hardware assisted unaligned access is mandatory, so
toolchains will not emit helper code and rather let the hardware do it's
job. 
Now the situation is as follows: hardware platforms without the ability
to do unaligned access in hardware will just work even though the code
is suboptimal, but users of an ARMv7 platform will encounter unexplained
system hangs, which is bad imho. This patch just makes behaviour
consistent across ARMv5 and ARMv7 platforms.

If we really want to disallow the use of unaligned memory operations in
U-Boot we should make all platforms fail at compile time, not make one
platform fail randomly at runtime. Do you think this is the way to go?
I'm fine either way, I'm just not okay with the current situation where
one platform fails while another just works.

Thanks,
Lucas

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Albert ARIBAUD]

Hi Aneesh,

> >> BTW, I agree that enabling un-aligned access is not a bad idea.
> >
> > Just being "not a bad idea" is not enough for me to accept this. It
> > will have to be the sole sound solution to a problem, and at this
> > point, I do not think it is as far as USB structure mis-alignement
> > issues are concerned.
> 
> My point is that enabling un-aligned accesses in itsown merit
> is not a bad idea, not as a solution to this problem. I have seen
> it being enabled in HLOS environment. TI's Symbian port for
> instance had it enabled for OMAP3. I don't
> know why Linux too shoudln't take advantage of such hw
> features. Perhaps you don't want to take it at this point of time to
> force the real solution to the USB problem, which is reasonable.

What is the (non-contrived) problem to which allowing mis-aligned
accesses would be a solution?

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Lucas Stach]

Hi Albert,

Am Montag, den 25.06.2012, 22:17 +0200 schrieb Albert ARIBAUD:
> Hi Lucas,
> 
> On Sun, 24 Jun 2012 08:30:19 +0200, Lucas Stach <dev@lynxeye.de> wrote:
> > Hi Albert,
> > 
> > Am Samstag, den 23.06.2012, 11:01 +0200 schrieb Albert ARIBAUD:
> > [snip]
> > > > >> But apart from this, we certainly have situations where we have
> > > > >> unaligned accesses that are justified and could not be removed.
> > > > >> [...]
> > > > >> I cannot see how enabling a hardware feature can be seen as
> > > > >> allowing of lax behaviour. As some of the USB structs are used
> > > > >> to access hardware registers, we can not align every struct
> > > > >> there.
> > > 
> > > If the access is in true RAM, then we can always realign the data;
> > > and I don't know of memory-mapped registers which would be
> > > unaligned wrt their width. If some USB controller is designed so,
> > > then the fix should only and explicitly affect that controller,
> > > because we don't know it it will always be used with an ARM
> > > implementation that can do unaligned accesses.
> > 
> > My point is: on ARM platforms that can't do unaligned access in
> > hardware the toolchain will automaticly emit helper code to work
> > around this. On an ARMv7 platform hardware assisted unaligned access
> > is mandatory, so toolchains will not emit helper code and rather let
> > the hardware do it's job.
> 
> My point is that there should be a reason for doing unaligned access to
> boot (pun half-intended) and I don't see any.
> 
I see your point here.

> > Now the situation is as follows: hardware platforms without the
> > ability to do unaligned access in hardware will just work even though
> > the code is suboptimal, but users of an ARMv7 platform will encounter
> > unexplained system hangs, which is bad imho. This patch just makes
> > behaviour consistent across ARMv5 and ARMv7 platforms.
> 
> Wouldn't aligning data properly make behaviour consistent as well?
> 
Consistent in the way of "it works on all platforms", but not in the way
of programming practice. ARMv7 people will have to care about alignment
to not encounter runtime errors, while on ARMv5 it just doesn't matter.
(Which certainly does not make it the right thing to do on ARMv5, but as
the existing USB code shows people just haven't cared about this.)

> > If we really want to disallow the use of unaligned memory operations
> > in U-Boot we should make all platforms fail at compile time, not make
> > one platform fail randomly at runtime. Do you think this is the way
> > to go? I'm fine either way, I'm just not okay with the current
> > situation where one platform fails while another just works.
> 
> I do not want to disallow unaligned accesses, because this assumes
> they were allowed in the first place; I want to keep not allowing them,
> just like they were unallowed far, and just like they can keep on
> being. If a misaligned access causes one platform to fail while another
> just works, then we should avoid the misaligned access by re-aligning
> it properly so as to make both platforms work.
> 
The thing I wanted to say is: if we generally consider unaligned access
a no-go for ARMv7 (which I'm fine with), we should equally consider it a
no-go for ARMv5. So I would rather want to see both platforms fail at
compile time if we have unaligned data, to avoid the unintended
introduction of such code in the codebase as has happened with the usb
code. Sadly I have not found a way to tell gcc to error out if
encounters an unaligned access. Currently ARMv5 "allows" unaligned data,
as toolchains insert helper code which makes it work.

Also it is really unfortunate that coding errors (which the unjustified
use of unaligned data certainly is) manifest as a runtime error, rather
than a compiletime failure, which make them a lot harder to spot.

> IOW, things will not change until I am shown some actual situation where
> a misaligned access is necessary and the core prevents it.
> 
I'm okay with you dropping this patch and will try to come up with
something that make programming constraints more consistent across both
ARM platforms.

Thanks,
Lucas

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Aneesh V]

Hi Albert,

On 06/25/2012 01:34 PM, Albert ARIBAUD wrote:
> Hi Aneesh,
>
>>>> BTW, I agree that enabling un-aligned access is not a bad idea.
>>>
>>> Just being "not a bad idea" is not enough for me to accept this. It
>>> will have to be the sole sound solution to a problem, and at this
>>> point, I do not think it is as far as USB structure mis-alignement
>>> issues are concerned.
>>
>> My point is that enabling un-aligned accesses in itsown merit
>> is not a bad idea, not as a solution to this problem. I have seen
>> it being enabled in HLOS environment. TI's Symbian port for
>> instance had it enabled for OMAP3. I don't
>> know why Linux too shoudln't take advantage of such hw
>> features. Perhaps you don't want to take it at this point of time to
>> force the real solution to the USB problem, which is reasonable.
>
> What is the (non-contrived) problem to which allowing mis-aligned
> accesses would be a solution?

memcpy() when there is a mismatch in the alignment of source and
destination buffers. Let's say the source buffer is 4 byte aligned
but the destination buffer is only 2 byte aligned. I believe relaxed
alignment requirements will help in writing an accelerated memcpy
routine for this case.

Again, my point is that as a platform software provider I would like
to enable such features to make maximum things work on my platform,
where as the developer of a generic sw module should probably avoid
depending on such features to ensure maximum portability.

br,
Aneesh

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Wolfgang Denk]

Dear Aneesh V,

In message <4FE8DCE7.7090700@ti.com> you wrote:
> 
> > What is the (non-contrived) problem to which allowing mis-aligned
> > accesses would be a solution?
> 
> memcpy() when there is a mismatch in the alignment of source and
> destination buffers. Let's say the source buffer is 4 byte aligned
> but the destination buffer is only 2 byte aligned. I believe relaxed
> alignment requirements will help in writing an accelerated memcpy
> routine for this case.

If memcpy() has problems with handling such a situation, then clearly
memcpy() needs fixing.

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
"I dislike companies that have a we-are-the-high-priests-of-hardware-
so-you'll-like-what-we-give-you attitude. I like commodity markets in
which iron-and-silicon hawkers know that they exist to  provide  fast
toys for software types like me to play with..."    - Eric S. Raymond

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Rob Herring]

On 06/22/2012 04:15 AM, Albert ARIBAUD wrote:
> Hi Lucas,
> 
> On Tue, 05 Jun 2012 21:06:20 +0200, Lucas Stach <dev@lynxeye.de> wrote:
>> Hi Stephen,
>>
>> Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
>>> On 06/05/2012 11:47 AM, Lucas Stach wrote:
>>>> Recent toolchains default to using the hardware feature for
>>>> unaligned access on ARM v7, rather than doing the software
>>>> fallback. According to ARM this is safe as all v7 implementations
>>>> have to support this feature.
>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
>>>>
>>>> To avoid CPU hangs when doing unaligned memory access, we have to
>>>> turn off alignment checking in our CPU initialisation code.
>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
>>>
>>> Does this behavior change trickle down to Linux/... too, or would
>>> an OS completely re-initialize this state, and hence not be
>>> affected?
>>>
>>
>> Linux in particular does reinitialize this state and I expect any
>> reasonable OS to do so.
> 
> Then what is the point of enabling it on U-Boot? Does it fix some issue
> whereby some mis-aligned piece of data cannot be properly aligned?
> 

This is a new optimization feature in gcc 4.7 (and backported to some
4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):

http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html

http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/

If you don't want to enable unaligned accesses, then
"-mno-unaligned-access" needs to be added.

Regards,
Rob

[U-Boot] [PATCH] arm: enable unaligned access on ARMv7

[Tetsuyuki Kobayashi]

Hi Rob and all,

>>>
>>> Am Dienstag, den 05.06.2012, 12:42 -0600 schrieb Stephen Warren:
>>>> On 06/05/2012 11:47 AM, Lucas Stach wrote:
>>>>> Recent toolchains default to using the hardware feature for
>>>>> unaligned access on ARM v7, rather than doing the software
>>>>> fallback. According to ARM this is safe as all v7 implementations
>>>>> have to support this feature.
>>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/BABJFFAE.html)
>>>>>
>>>>> To avoid CPU hangs when doing unaligned memory access, we have to
>>>>> turn off alignment checking in our CPU initialisation code.
>>>>> (http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0491c/CIHCGCFD.html)
>>>>
>>>> Does this behavior change trickle down to Linux/... too, or would
>>>> an OS completely re-initialize this state, and hence not be
>>>> affected?
>>>>
>>>
>>> Linux in particular does reinitialize this state and I expect any
>>> reasonable OS to do so.
>>
>> Then what is the point of enabling it on U-Boot? Does it fix some issue
>> whereby some mis-aligned piece of data cannot be properly aligned?
>>
> 
> This is a new optimization feature in gcc 4.7 (and backported to some
> 4.6 versions like the ubuntu 12.04 arm cross compiler (4.6.3)):
> 
> http://lists.linaro.org/pipermail/linaro-dev/2012-June/012360.html
> 
> http://seabright.co.nz/2012/06/11/kernel-not-booting-with-linaro-gcc/
> 
> If you don't want to enable unaligned accesses, then
> "-mno-unaligned-access" needs to be added.
> 

I verified it. Option "-mno-unaligned-access" works good.


include/mtd/cfi_flash.h

/* CFI standard query structure */
struct cfi_qry {
	u8	qry[3];
	u16	p_id;  <-- unaligned!
	...
} __attribute__((packed));


$ ${CROSS_COMPILE}gcc --version
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.6.2 20110921 (release) [ARM/embedded-4_6-branch revision 182083]
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

=====================================================================
Compiled without --mno-unaligned-access
$ ${CROSS_COMPILE}objdump -d -S u-boot 

                info->vendor = le16_to_cpu(qry.p_id);
    cc88:       e3003a1c        movw    r3, #2588       ; 0xa1c
    cc8c:       e1dd11bb        ldrh    r1, [sp, #27]  <-- this is unaligned access
    cc90:       ...
    cc94:       e18410b3        strh    r1, [r4, r3]


=====================================================================
Compiled with --mno-unaligned-access
$ ${CROSS_COMPILE}objdump -d -S u-boot 


                info->vendor = le16_to_cpu(qry.p_id);
    cce8:       e5dd101c        ldrb    r1, [sp, #28]  <--
    ccec:       e5dd301b        ldrb    r3, [sp, #27]  <-- separated 2 byte accesses
    ccf0:       ...
    ccf4:       e1831401        orr     r1, r3, r1, lsl #8
    ccf8:       e3003a1c        movw    r3, #2588       ; 0xa1c
    ccfc:       e18410b3        strh    r1, [r4, r3]

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Tetsuyuki Kobayashi]

Recent compiler generates unaligned memory access in armv7 default.
But current U-Boot does not allow unaligned memory access, so it causes
data abort exception.
This patch add compile option "-mno-unaligned-access" if it is available.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
---
 arch/arm/cpu/armv7/config.mk |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/cpu/armv7/config.mk b/arch/arm/cpu/armv7/config.mk
index 5407cb6..560c084 100644
--- a/arch/arm/cpu/armv7/config.mk
+++ b/arch/arm/cpu/armv7/config.mk
@@ -26,6 +26,8 @@ PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float
 # supported by more tool-chains
 PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5)
 PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7)
+PF_CPPFLAGS_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,)
+PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
 
 # =========================================================================
 #
-- 1.7.9.5 

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Måns Rullgård]

Tetsuyuki Kobayashi <koba@kmckk.co.jp> writes:

> Recent compiler generates unaligned memory access in armv7 default.
> But current U-Boot does not allow unaligned memory access, so it causes
> data abort exception.
> This patch add compile option "-mno-unaligned-access" if it is available.

Why not allow unaligned accesses instead?

-- 
M?ns Rullg?rd
mans at mansr.com

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Lucas Stach]

Am Montag, den 02.07.2012, 10:53 +0100 schrieb M?ns Rullg?rd:
> Tetsuyuki Kobayashi <koba@kmckk.co.jp> writes:
> 
> > Recent compiler generates unaligned memory access in armv7 default.
> > But current U-Boot does not allow unaligned memory access, so it causes
> > data abort exception.
> > This patch add compile option "-mno-unaligned-access" if it is available.
> 
> Why not allow unaligned accesses instead?
> 
IMHO, our recent discussion showed that both ways are wrong.
"-mno-unaligned-access" works around misaligned data on the software
level, while allowing unaligned access does on the hardware level.

What we really want is no unaligned access in U-Boot at all. Just
because "-mno-unaligned-access" is the default on ARMv5, we should not
consider it a gold standard.

Thanks,
Lucas

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Måns Rullgård]

Lucas Stach <dev@lynxeye.de> writes:

> Am Montag, den 02.07.2012, 10:53 +0100 schrieb M?ns Rullg?rd:
>> Tetsuyuki Kobayashi <koba@kmckk.co.jp> writes:
>> 
>> > Recent compiler generates unaligned memory access in armv7 default.
>> > But current U-Boot does not allow unaligned memory access, so it causes
>> > data abort exception.
>> > This patch add compile option "-mno-unaligned-access" if it is available.
>> 
>> Why not allow unaligned accesses instead?
>> 
> IMHO, our recent discussion showed that both ways are wrong.
> "-mno-unaligned-access" works around misaligned data on the software
> level, while allowing unaligned access does on the hardware level.
>
> What we really want is no unaligned access in U-Boot at all. Just
> because "-mno-unaligned-access" is the default on ARMv5, we should not
> consider it a gold standard.

It's slightly more complicated than that.  Data can be misaligned for a
variety of reasons:

1. Errors in software.
2. Specified by a file format or communication protocol.
3. Deliberately misaligned by the compiler.

Misaligned data of type 1 should of course be fixed properly, not worked
around in any way.

Type 2 happens all the time, and has to be dealt with one way or
another.  If the hardware supports unaligned accesses, this is usually
faster than reading a byte at a time.

When targeting ARMv6 and later, recent gcc versions have started issuing
deliberate unaligned accesses where previously byte by byte accesses
would have been done.  This happens with "packed" structs and sometimes
to write multiple smaller values at once, typically when
zero-initialising things.  These unaligned accesses are *good*.  They
make code smaller and faster.

The real problem here is that u-boot is setting the strict alignment
checking flag, invalidating the assumption of the compiler that the
system allows unaligned accesses.  For ARMv5 and earlier, setting this
flag is usually advisable since it makes finding accidental unaligned
accesses much easier.

This was debated in the context of the kernel a while ago, ultimately
leading to strict alignment being disabled for ARMv6 and up [1].

[1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05

-- 
M?ns Rullg?rd
mans at mansr.com

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Tetsuyuki Kobayashi]

Hello, M?ns
Thank you for summarizing.

I am not against you.
I'm OK either "Allow unaligned access in U-Boot setting" or
"Specify compiler not to generate unaligned memory access"
or other.
I just want to solve hung-up by unaligned access.
I follow custodian's decision.

(2012/07/03 1:14), M?ns Rullg?rd wrote:
> Lucas Stach<dev@lynxeye.de>  writes:
>
>> Am Montag, den 02.07.2012, 10:53 +0100 schrieb M?ns Rullg?rd:
>>> Tetsuyuki Kobayashi<koba@kmckk.co.jp>  writes:
>>>
>>>> Recent compiler generates unaligned memory access in armv7 default.
>>>> But current U-Boot does not allow unaligned memory access, so it causes
>>>> data abort exception.
>>>> This patch add compile option "-mno-unaligned-access" if it is available.
>>>
>>> Why not allow unaligned accesses instead?
>>>
>> IMHO, our recent discussion showed that both ways are wrong.
>> "-mno-unaligned-access" works around misaligned data on the software
>> level, while allowing unaligned access does on the hardware level.
>>
>> What we really want is no unaligned access in U-Boot at all. Just
>> because "-mno-unaligned-access" is the default on ARMv5, we should not
>> consider it a gold standard.
>
> It's slightly more complicated than that.  Data can be misaligned for a
> variety of reasons:
>
> 1. Errors in software.
> 2. Specified by a file format or communication protocol.
> 3. Deliberately misaligned by the compiler.
>
> Misaligned data of type 1 should of course be fixed properly, not worked
> around in any way.
>
> Type 2 happens all the time, and has to be dealt with one way or
> another.  If the hardware supports unaligned accesses, this is usually
> faster than reading a byte at a time.
>
> When targeting ARMv6 and later, recent gcc versions have started issuing
> deliberate unaligned accesses where previously byte by byte accesses
> would have been done.  This happens with "packed" structs and sometimes
> to write multiple smaller values at once, typically when
> zero-initialising things.  These unaligned accesses are *good*.  They
> make code smaller and faster.
>
> The real problem here is that u-boot is setting the strict alignment
> checking flag, invalidating the assumption of the compiler that the
> system allows unaligned accesses.  For ARMv5 and earlier, setting this
> flag is usually advisable since it makes finding accidental unaligned
> accesses much easier.
>
> This was debated in the context of the kernel a while ago, ultimately
> leading to strict alignment being disabled for ARMv6 and up [1].
>
> [1] http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05
>

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Albert ARIBAUD]

Hi M?ns,

On Mon, 02 Jul 2012 17:14:40 +0100, M?ns Rullg?rd <mans@mansr.com>
wrote:

> > IMHO, our recent discussion showed that both ways are wrong.
> > "-mno-unaligned-access" works around misaligned data on the software
> > level, while allowing unaligned access does on the hardware level.
> >
> > What we really want is no unaligned access in U-Boot at all. Just
> > because "-mno-unaligned-access" is the default on ARMv5, we should
> > not consider it a gold standard.
> 
> It's slightly more complicated than that.  Data can be misaligned for
> a variety of reasons:
> 
> 1. Errors in software.
> 2. Specified by a file format or communication protocol.
> 3. Deliberately misaligned by the compiler.
> 
> Misaligned data of type 1 should of course be fixed properly, not
> worked around in any way.

Agreed.

> Type 2 happens all the time, and has to be dealt with one way or
> another.  If the hardware supports unaligned accesses, this is usually
> faster than reading a byte at a time.

Sorry but I don't accept a systemic change as a good solution to a
specific issue. The best fix is "don't accept using a protocol or file
format which was not designed to avoid native misaligned accesses", and
the second best fix is "if you really must deal with a protocol or file
format which causes native alignment issues, then the fix should only
affect the protocol or file format's issue, not with your system which
is not the root cause".

But to be honest, I haven't seen such badly a designed protocol or file
format; their designers tend to make sure that (assuming the start of a
protocol or file 'content' (frame, record, etc) is aligned, then all
fields in it are aligned as well. Can someone point me to an example of
a protocol or file format which does present such a misalignment risk ?

> When targeting ARMv6 and later, recent gcc versions have started
> issuing deliberate unaligned accesses where previously byte by byte
> accesses would have been done.

Wrongly formulated: "mis-aligning deliberately" seems to imply that
GCC did away with the possibility of properly aligning, which they of
course did not. What they did is switch its default behavior regarding
native misaligned accesses from forbidding to allowing them. The change
here is of policy, a change which we may or may not want to follow. 

> This happens with "packed" structs
> and sometimes to write multiple smaller values at once, typically when
> zero-initialising things.  These unaligned accesses are *good*.  They
> make code smaller and faster.

Again, "good" is a policy, or subjective, statement, not an absolute.
Just as "good" is correctly aligning data in the first place (to begin
with, in protocols and file formats) and keeping the compiler's native
misaligned access policy set to "do not allow".

> The real problem here is that u-boot is setting the strict alignment
> checking flag, invalidating the assumption of the compiler that the
> system allows unaligned accesses.  For ARMv5 and earlier, setting this
> flag is usually advisable since it makes finding accidental unaligned
> accesses much easier.

Just as it is for ARMv6 and up. Again, just because the compiler folks
changed their default policy does not mean we should change ours,
which is not based on the same goals. 

> This was debated in the context of the kernel a while ago, ultimately
> leading to strict alignment being disabled for ARMv6 and up [1].
> 
> [1]
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05

I'd rather have a link to the rationale than to the commit, but anyway,
the kernel folks' decision is theirs and does not necessarily apply to
us. I have mailed Catalin Marinas off-list to get details on the
rationale and context of the kernel patch; I will report conclusions
here.

Meanwhile, our policy regarding misalignment accesses is to only allow
them when externally required (by something other than a bad design).
Someone please show me such an external requirement for U-Boot, and I
will reconsider -- and then, other arch custodians may have a problem
with that too.

Regarding the origin of this patch, i.e. a mis-alignment of USB fields,
and unless U-Boot USB folks say otherwise, this issue should be fixed
by aligning said fields properly.

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Gary Thomas]

On 2012-07-02 03:42, Tetsuyuki Kobayashi wrote:
> Recent compiler generates unaligned memory access in armv7 default.
> But current U-Boot does not allow unaligned memory access, so it causes
> data abort exception.
> This patch add compile option "-mno-unaligned-access" if it is available.
>
> Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
> ---
>   arch/arm/cpu/armv7/config.mk |    2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm/cpu/armv7/config.mk b/arch/arm/cpu/armv7/config.mk
> index 5407cb6..560c084 100644
> --- a/arch/arm/cpu/armv7/config.mk
> +++ b/arch/arm/cpu/armv7/config.mk
> @@ -26,6 +26,8 @@ PLATFORM_RELFLAGS += -fno-common -ffixed-r8 -msoft-float
>   # supported by more tool-chains
>   PF_CPPFLAGS_ARMV7 := $(call cc-option, -march=armv7-a, -march=armv5)
>   PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_ARMV7)
> +PF_CPPFLAGS_NO_UNALIGNED := $(call cc-option, -mno-unaligned-access,)
> +PLATFORM_CPPFLAGS += $(PF_CPPFLAGS_NO_UNALIGNED)
>
>   # =========================================================================
>   #
> -- 1.7.9.5

Tested-by: Gary Thomas <gary@mlbassoc.com>

-- 
------------------------------------------------------------
Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
------------------------------------------------------------

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Albert ARIBAUD]

on Thu, 5 Jul 2012 09:57:19 +0200,
Albert ARIBAUD <albert.u.boot@aribaud.net> wrote :

> Hi M?ns,
> 
> On Mon, 02 Jul 2012 17:14:40 +0100, M?ns Rullg?rd <mans@mansr.com>
> wrote:
> 
> > > IMHO, our recent discussion showed that both ways are wrong.
> > > "-mno-unaligned-access" works around misaligned data on the software
> > > level, while allowing unaligned access does on the hardware level.
> > >
> > > What we really want is no unaligned access in U-Boot at all. Just
> > > because "-mno-unaligned-access" is the default on ARMv5, we should
> > > not consider it a gold standard.
> > 
> > It's slightly more complicated than that.  Data can be misaligned for
> > a variety of reasons:
> > 
> > 1. Errors in software.
> > 2. Specified by a file format or communication protocol.
> > 3. Deliberately misaligned by the compiler.
> > 
> > Misaligned data of type 1 should of course be fixed properly, not
> > worked around in any way.
> 
> Agreed.
> 
> > Type 2 happens all the time, and has to be dealt with one way or
> > another.  If the hardware supports unaligned accesses, this is usually
> > faster than reading a byte at a time.
> 
> Sorry but I don't accept a systemic change as a good solution to a
> specific issue. The best fix is "don't accept using a protocol or file
> format which was not designed to avoid native misaligned accesses", and
> the second best fix is "if you really must deal with a protocol or file
> format which causes native alignment issues, then the fix should only
> affect the protocol or file format's issue, not with your system which
> is not the root cause".
> 
> But to be honest, I haven't seen such badly a designed protocol or file
> format; their designers tend to make sure that (assuming the start of a
> protocol or file 'content' (frame, record, etc) is aligned, then all
> fields in it are aligned as well. Can someone point me to an example of
> a protocol or file format which does present such a misalignment risk ?
> 
> > When targeting ARMv6 and later, recent gcc versions have started
> > issuing deliberate unaligned accesses where previously byte by byte
> > accesses would have been done.
> 
> Wrongly formulated: "mis-aligning deliberately" seems to imply that
> GCC did away with the possibility of properly aligning, which they of
> course did not. What they did is switch its default behavior regarding
> native misaligned accesses from forbidding to allowing them. The change
> here is of policy, a change which we may or may not want to follow. 
> 
> > This happens with "packed" structs
> > and sometimes to write multiple smaller values at once, typically when
> > zero-initialising things.  These unaligned accesses are *good*.  They
> > make code smaller and faster.
> 
> Again, "good" is a policy, or subjective, statement, not an absolute.
> Just as "good" is correctly aligning data in the first place (to begin
> with, in protocols and file formats) and keeping the compiler's native
> misaligned access policy set to "do not allow".
> 
> > The real problem here is that u-boot is setting the strict alignment
> > checking flag, invalidating the assumption of the compiler that the
> > system allows unaligned accesses.  For ARMv5 and earlier, setting this
> > flag is usually advisable since it makes finding accidental unaligned
> > accesses much easier.
> 
> Just as it is for ARMv6 and up. Again, just because the compiler folks
> changed their default policy does not mean we should change ours,
> which is not based on the same goals. 
> 
> > This was debated in the context of the kernel a while ago, ultimately
> > leading to strict alignment being disabled for ARMv6 and up [1].
> > 
> > [1]
> > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=8428e84d42179c2a00f5f6450866e70d802d1d05
> 
> I'd rather have a link to the rationale than to the commit, but anyway,
> the kernel folks' decision is theirs and does not necessarily apply to
> us. I have mailed Catalin Marinas off-list to get details on the
> rationale and context of the kernel patch; I will report conclusions
> here.
> 
> Meanwhile, our policy regarding misalignment accesses is to only allow
> them when externally required (by something other than a bad design).
> Someone please show me such an external requirement for U-Boot, and I
> will reconsider -- and then, other arch custodians may have a problem
> with that too.
> 
> Regarding the origin of this patch, i.e. a mis-alignment of USB fields,
> and unless U-Boot USB folks say otherwise, this issue should be fixed
> by aligning said fields properly.

We are nearing the release, and we obviously won't have misalignments
fixed and tested in time for it.

So I suspect that if I want the ARM U-Boot release to work I'll have to
allow this patch in my master branch to be pulled in for 12.07, so that
the compiler keeps behaving as it did before gcc changed the default.

... but only for the upcoming release, i.e. I will revert the patch in
my 'next' branch, which will apply right after 12.07 is out. Therefore,
before next release, misalignments will have to be fixed at the root.

Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Mike Frysinger]

On Monday 02 July 2012 12:14:40 M?ns Rullg?rd wrote:
> It's slightly more complicated than that.  Data can be misaligned for a
> variety of reasons:
> 
> 1. Errors in software.
> 2. Specified by a file format or communication protocol.
> 3. Deliberately misaligned by the compiler.
> 
> Misaligned data of type 1 should of course be fixed properly, not worked
> around in any way.

it's also a reliability aspect.  people don't write bug free software, not bug 
free protocols, nor bug free compilers.  when misalignment does happen in the 
field, it's a hell of a lot better if the software continued to execute 
correctly rather than randomly triggered an exception.
-mike
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[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Mike Frysinger]

On Thursday 05 July 2012 03:57:19 Albert ARIBAUD wrote:
> But to be honest, I haven't seen such badly a designed protocol or file
> format; their designers tend to make sure that (assuming the start of a
> protocol or file 'content' (frame, record, etc) is aligned, then all
> fields in it are aligned as well. Can someone point me to an example of
> a protocol or file format which does present such a misalignment risk ?

simply search the kernel for get_unaligned then.  there are plenty of examples 
in there.  granted, many apply to stacks that don't show up in u-boot (yet?) 
such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few 
edge cases too.
-mike
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[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Albert ARIBAUD]

Hi Mike,

On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger <vapier@gentoo.org> wrote:
> On Monday 02 July 2012 12:14:40 M?ns Rullg?rd wrote:
> > It's slightly more complicated than that.  Data can be misaligned for a
> > variety of reasons:
> > 
> > 1. Errors in software.
> > 2. Specified by a file format or communication protocol.
> > 3. Deliberately misaligned by the compiler.
> > 
> > Misaligned data of type 1 should of course be fixed properly, not worked
> > around in any way.
> 
> it's also a reliability aspect.  people don't write bug free software, not bug 
> free protocols, nor bug free compilers.  when misalignment does happen in the 
> field, it's a hell of a lot better if the software continued to execute 
> correctly rather than randomly triggered an exception.
> -mike

Nitpick: this is robustness, not reliability.

That being said, yes, this robustness is desirable when you do not control all
of the SW running on the product; Linux, for instance, will have to execute
processes which were built with any old (or new) compiler settings, thus the
Linux folks have to make sure the kernel won't fail running those.

But the only uncontrolled SW U-Boot runs is its payload -- typically the
kernel image -- which are usually very cautious in what they assume they can
do, thus are unlikely to perform unaligned accesses.

(pasting your other comment)

> simply search the kernel for get_unaligned then.  there are plenty of examples 
> in there.  granted, many apply to stacks that don't show up in u-boot (yet?) 
> such as bluetooth, wireless, and irda, but i'm pretty sure TCP/IPv4 has a few 
> edge cases too.

I'll have a look, if only to lament that protocol are not what they used to be
in the old days. :)

Anyway: as I said: performing *controlled* unaligned accesses for external reasons
other than bugs is fine with me. Having our own get_unaligned() in such places
would be fine with me.
 
Amicalement,
-- 
Albert.

[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Mike Frysinger]

On Thursday 19 July 2012 02:28:05 Albert ARIBAUD wrote:
> On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger wrote:
> > On Monday 02 July 2012 12:14:40 M?ns Rullg?rd wrote:
> > > It's slightly more complicated than that.  Data can be misaligned for a
> > > variety of reasons:
> > > 
> > > 1. Errors in software.
> > > 2. Specified by a file format or communication protocol.
> > > 3. Deliberately misaligned by the compiler.
> > > 
> > > Misaligned data of type 1 should of course be fixed properly, not
> > > worked around in any way.
> > 
> > it's also a reliability aspect.  people don't write bug free software,
> > not bug free protocols, nor bug free compilers.  when misalignment does
> > happen in the field, it's a hell of a lot better if the software
> > continued to execute correctly rather than randomly triggered an
> > exception.
> 
> Nitpick: this is robustness, not reliability.

useless pedantry: by increasing robustness, the system is more reliable

> That being said, yes, this robustness is desirable when you do not control
> all of the SW running on the product; Linux, for instance, will have to
> execute processes which were built with any old (or new) compiler
> settings, thus the Linux folks have to make sure the kernel won't fail
> running those.
> 
> But the only uncontrolled SW U-Boot runs is its payload -- typically the
> kernel image -- which are usually very cautious in what they assume they
> can do, thus are unlikely to perform unaligned accesses.

it isn't just that.  there is no way you can guarantee both the linux kernel 
and u-boot code bases themselves are perfect.  in fact, it's even worse when 
these are the ones that get tripped up because it means your system 
resets/hardlocks/kills a kitten.  when doing driver development under the 
linux kernel, we would come across parts of core stacks that lacked alignment 
checking and would panic the system.  sometimes it would always panic, other 
times it depended on factors that made life worse: the compiler version (newer 
ones always like to pack/optimize better), the actual data stream, or the 
execution paths.

> > simply search the kernel for get_unaligned then.  there are plenty of
> > examples in there.  granted, many apply to stacks that don't show up in
> > u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure
> > TCP/IPv4 has a few edge cases too.
> 
> I'll have a look, if only to lament that protocol are not what they used to
> be in the old days. :)
> 
> Anyway: as I said: performing *controlled* unaligned accesses for external
> reasons other than bugs is fine with me. Having our own get_unaligned() in
> such places would be fine with me.

i have no problem adding put/get_unaligned() to all the right places.  that 
makes perfect sense.  but, as an orthogonal issue wrt ARMv7, i don't see any 
problem enabling hardware functionality: it increases robustness (:P), shrinks 
the code base (all the get/put unaligned macros expand into a single memory 
access as they no longer have to do alignment fixups in software), and speeds 
up the runtime (a single unaligned memory access is always faster than address 
masking/multiple loads/bit shifting/etc... -- obviously this ignores 
multimedia type code that does alignment adjustment at the start, then lets of 
memory accesses, then another adjustment at the end, but that's not what we're 
talking about here).

if you want to tell people that if they found an unaligned access in code they 
must fix that, then great.  make them fix it.  then once that bug has been fixed, 
let's merge the purely optimization patch that allows the hardware to do 
unaligned accesses.
-mike
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[U-Boot] [PATCH] arm: armv7: add compile option -mno-unaligned-access if available

[Albert ARIBAUD]

Hi Mike,

On Thu, 19 Jul 2012 10:27:07 -0400, Mike Frysinger <vapier@gentoo.org> wrote:
> On Thursday 19 July 2012 02:28:05 Albert ARIBAUD wrote:
> > On Thu, 19 Jul 2012 00:29:23 -0400, Mike Frysinger wrote:
> > > On Monday 02 July 2012 12:14:40 M?ns Rullg?rd wrote:
> > > > It's slightly more complicated than that.  Data can be misaligned for a
> > > > variety of reasons:
> > > > 
> > > > 1. Errors in software.
> > > > 2. Specified by a file format or communication protocol.
> > > > 3. Deliberately misaligned by the compiler.
> > > > 
> > > > Misaligned data of type 1 should of course be fixed properly, not
> > > > worked around in any way.
> > > 
> > > it's also a reliability aspect.  people don't write bug free software,
> > > not bug free protocols, nor bug free compilers.  when misalignment does
> > > happen in the field, it's a hell of a lot better if the software
> > > continued to execute correctly rather than randomly triggered an
> > > exception.
> > 
> > Nitpick: this is robustness, not reliability.
> 
> useless pedantry: by increasing robustness, the system is more reliable

Your description confirms that robustness and reliability are not equivalent,
thereby goes against your statement about pedantry... :)
 
> > That being said, yes, this robustness is desirable when you do not control
> > all of the SW running on the product; Linux, for instance, will have to
> > execute processes which were built with any old (or new) compiler
> > settings, thus the Linux folks have to make sure the kernel won't fail
> > running those.
> > 
> > But the only uncontrolled SW U-Boot runs is its payload -- typically the
> > kernel image -- which are usually very cautious in what they assume they
> > can do, thus are unlikely to perform unaligned accesses.
> 
> it isn't just that.  there is no way you can guarantee both the linux kernel 
> and u-boot code bases themselves are perfect.  in fact, it's even worse when 
> these are the ones that get tripped up because it means your system 
> resets/hardlocks/kills a kitten.  when doing driver development under the 
> linux kernel, we would come across parts of core stacks that lacked alignment 
> checking and would panic the system.  sometimes it would always panic, other 
> times it depended on factors that made life worse: the compiler version (newer 
> ones always like to pack/optimize better), the actual data stream, or the 
> execution paths.

Correct; here I was considering the requirements / operating conditions for both
projects, I was not considering development issues -- bugs during development
happen (morethan they do in the field, hopefully).

Do you mean you'd like to catch misalignments as early as possible, and would
like to have both -munaligned-access and A=1 during dev?
 
> > > simply search the kernel for get_unaligned then.  there are plenty of
> > > examples in there.  granted, many apply to stacks that don't show up in
> > > u-boot (yet?) such as bluetooth, wireless, and irda, but i'm pretty sure
> > > TCP/IPv4 has a few edge cases too.
> > 
> > I'll have a look, if only to lament that protocol are not what they used to
> > be in the old days. :)
> > 
> > Anyway: as I said: performing *controlled* unaligned accesses for external
> > reasons other than bugs is fine with me. Having our own get_unaligned() in
> > such places would be fine with me.
> 
> i have no problem adding put/get_unaligned() to all the right places.  that 
> makes perfect sense.  but, as an orthogonal issue wrt ARMv7, i don't see any 
> problem enabling hardware functionality: it increases robustness (:P), shrinks 
> the code base (all the get/put unaligned macros expand into a single memory 
> access as they no longer have to do alignment fixups in software), and speeds 
> up the runtime (a single unaligned memory access is always faster than address 
> masking/multiple loads/bit shifting/etc... -- obviously this ignores 
> multimedia type code that does alignment adjustment at the start, then lets of 
> memory accesses, then another adjustment at the end, but that's not what we're 
> talking about here).

I wouldn't care about the ARMv7 implementing explicit unaligned accesses with
native instructions if it didn't mean it won't catch unwanted unaligned accesses
any more, and thus such unalignments will be found in another context and will
take some more time to trace back.  

> if you want to tell people that if they found an unaligned access in code they 
> must fix that, then great.  make them fix it.  then once that bug has been fixed, 
> let's merge the purely optimization patch that allows the hardware to do 
> unaligned accesses.

My problem is that as long as people start configuring their HW and compiler to
not care about unaligned accesses, they *won't* find such accesses when accidental,
because nothing will tell them.

Here's my suggestion: when building U-Boot as usual, strict aligment policy is
enforced, i.e. -mno-unaligned-access and A=1, and for platforms that could benefit
from native unaligned accesses, a run-time warning is emitted on the console.
However, with a specific command 'PRODUCTION' line option added, the constraint
is relaxed, i.e. -munaligned-access and A=0, no run-time message, but a build
warning is emitted stating that this build is afoul of the U-Boot strict policy.

This way, you get the robustness you want as you can easily build an ARMv7-
efficient binary, and I get the one I want as the build used for development is
slightly less MCPS-efficient but will catch potential issues.

Comments welcome.

> -mike

Amicalement,
-- 
Albert.