Re: [PATCH] ARM: /proc/cpuinfo: Use DT machine name when possible

["Pali Rohár" <pali.rohar@gmail.com>]

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On Wednesday 18 June 2014 22:46:19 Rob Herring wrote:
> On Wed, Jun 18, 2014 at 2:22 PM, Pali Rohár 
<pali.rohar@gmail.com> wrote:
> > On Wednesday 18 June 2014 21:07:35 Rob Herring wrote:
> >> On Wed, Jun 18, 2014 at 11:54 AM, Pali Rohár
> > 
> > <pali.rohar@gmail.com> wrote:
> >> > Machine name from board description is some generic name
> >> > on DT kernel. DT provides machine name property which is
> >> > specific for board, so use it instead generic one when
> >> > possible.
> >> > 
> >> > Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
> >> > ---
> >> > 
> >> >  arch/arm/kernel/setup.c |    7 +++++--
> >> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >> > 
> >> > diff --git a/arch/arm/kernel/setup.c
> >> > b/arch/arm/kernel/setup.c index 8a16ee5..fbc7b4f 100644
> >> > --- a/arch/arm/kernel/setup.c
> >> > +++ b/arch/arm/kernel/setup.c
> >> > @@ -875,10 +875,13 @@ void __init setup_arch(char
> >> > **cmdline_p)
> >> > 
> >> >         setup_processor();
> >> >         mdesc = setup_machine_fdt(__atags_pointer);
> >> > 
> >> > -       if (!mdesc)
> >> > +       if (mdesc)
> >> > +               machine_name =
> >> > of_flat_dt_get_machine_name(); +       else
> >> > 
> >> >                 mdesc =
> >> >                 setup_machine_tags(__atags_pointer,
> >> >                 __machine_arch_type);
> >> >         
> >> >         machine_desc = mdesc;
> >> > 
> >> > -       machine_name = mdesc->name;
> >> > +       if (!machine_name)
> >> > +               machine_name = mdesc->name;
> >> > 
> >> >         if (mdesc->reboot_mode != REBOOT_HARD)
> >> >         
> >> >                 reboot_mode = mdesc->reboot_mode;
> >> 
> >> I did a similar patch previously[1]. Like my original
> >> patch, your patch unconditionally changes the name which
> >> could be considered part of the userspace ABI. It's
> >> arguably not good practice for userspace to depend on the
> >> name, but there are likely cases that do. So I think this
> >> needs to be optional and only use the DT name if the
> >> machine desc name is NULL.
> >> 
> >> So something like the below and then change the machine
> >> descriptors you care about. The default generic DT machine
> >> desc should definitely be changed.
> >> 
> >> I had a follow-up discussion with Grant about his concerns
> >> in the thread about knowing which machine desc used to
> >> boot. He said he was okay if just the machine desc address
> >> is printed out.
> >> 
> >> 
> >> diff --git a/arch/arm/kernel/setup.c
> >> b/arch/arm/kernel/setup.c index 50e198c..1479250 100644
> >> --- a/arch/arm/kernel/setup.c
> >> +++ b/arch/arm/kernel/setup.c
> >> @@ -887,7 +887,7 @@ void __init setup_arch(char
> >> **cmdline_p)
> >> 
> >>         if (!mdesc)
> >>         
> >>                 mdesc = setup_machine_tags(__atags_pointer,
> >> 
> >> __machine_arch_type);
> >> 
> >>         machine_desc = mdesc;
> >> 
> >> -       machine_name = mdesc->name;
> >> +       machine_name = mdesc->name ? mdesc->name :
> >> of_flat_dt_get_machine_name();
> >> 
> >>         if (mdesc->reboot_mode != REBOOT_HARD)
> >>         
> >>                 reboot_mode = mdesc->reboot_mode;
> >> 
> >> [1]
> >> http://lists.infradead.org/pipermail/linux-arm-kernel/2013-> >> No vember/208878.html
> > 
> > Hi,
> > 
> > now legacy board code for Nokia N900 (RX-51) is migrating to
> > DT kernel and there is problem with info which
> > /proc/cpuinfo reports
> 
> Thanks for providing your motivation which was missing from
> the commit message.
> 
> > New DT kernel (3.16-rc1) reports:
> > 
> > # busybox cat /proc/cpuinfo
> > processor       : 0
> > model name      : ARMv7 Processor rev 3 (v7l)
> > Features        : swp half thumb fastmult vfp edsp thumbee
> > neon vfpv3 tls vfpd32
> > CPU implementer : 0x41
> > CPU architecture: 7
> > CPU variant     : 0x2
> > CPU part        : 0xc08
> > CPU revision    : 3
> > 
> > Hardware        : Generic OMAP3 (Flattened Device Tree)
> > Revision        : 0000
> > Serial          : 0000000000000000
> > 
> > But legacy board code kernel reports:
> > 
> > # busybox cat /proc/cpuinfo
> > processor       : 0
> > model name      : ARMv7 Processor rev 3 (v7l)
> > Features        : swp half thumb fastmult vfp edsp thumbee
> > neon vfpv3 tls vfpd32
> > CPU implementer : 0x41
> > CPU architecture: 7
> > CPU variant     : 0x2
> > CPU part        : 0xc08
> > CPU revision    : 3
> > 
> > Hardware        : Nokia RX-51 board
> > Revision        : 0012
> > Serial          : 0000000000000000
> > 
> > Basically in DT kernel is missing Hardware, Revision and
> > probably also Serial key. (Now I used only qemu for testing
> > which set serial key to 0). All these informations is used
> > by userspace applications which determinate how to behave.
> 
> It is somewhat fragile to expect the name in the DT to match
> the old name from the kernel. As your patch to n900 dts
> shows, we'd probably have to update every dts file to make
> them match. While I think we should work to remove this
> string from the kernel, userspace depending on the DT model
> string is a bad idea. For example, if you had some platform
> with multiple OEMs just rebranding the same base design, they
> may all want to put their own model string into each product.
> The true h/w id is the compatible string.
> 
> Serial number is easy. There is already a standard although
> not widely used property for it with "/serial-number". You
> just need to wire it up to cpuinfo.
> 

There is no "/serial-number" property in kernel:

$ ls -l -a /sys/bus/soc/devices/soc0/
drwxr-xr-x    3 0        0                0 Jan  1 00:00 .
drwxr-xr-x   18 0        0                0 Jan  1 00:00 ..
-r--r--r--    1 0        0             4096 Jan  1 00:00 family
-r--r--r--    1 0        0             4096 Jan  1 00:00 machine
drwxr-xr-x    2 0        0                0 Jan  1 00:00 power
-r--r--r--    1 0        0             4096 Jan  1 00:00 revision
lrwxrwxrwx    1 0        0                0 Jan  1 00:00 
subsystem -> ../../bus/soc
-r--r--r--    1 0        0             4096 Jan  1 00:00 type
-rw-r--r--    1 0        0             4096 Jan  1 00:00 uevent

> > So without this patch DT migration for Nokia N900 cannot be
> > done without breaking userspace - which is not
> > acceptable...
> 
> I agree, but I would like to come up with something that
> prevents future dependence on this string.
> 
> > Also I still did not know why DT kernel does not report
> > Revision number which is passed by bootloader via atags.
> > Any idea?
> 
> Probably because no one cared until now and revision info for
> every SOC is different. I would like to see revision info set
> in the DT in a standard way and remove the various SOC
> specific kernel implementations.
> 
> Rob

-- 
Pali Rohár
pali.rohar@gmail.com

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