[PATCH] ARM: Qualify enabling of swiotlb_init()

[PATCH] ARM: Qualify enabling of swiotlb_init()

[Florian Fainelli]

We do not need a SWIOTLB unless we have DRAM that is addressable beyond
the arm_dma_limit. Compare max_pfn with arm_dma_pfn_limit to determine
whether we do need a SWIOTLB to be initialized.

Fixes: ad3c7b18c5b3 ("arm: use swiotlb for bounce buffering on LPAE configs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
---
 arch/arm/mm/init.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
index 828a2561b229..8356bf1daa28 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -301,7 +301,11 @@ static void __init free_highpages(void)
 void __init mem_init(void)
 {
 #ifdef CONFIG_ARM_LPAE
-	swiotlb_init(1);
+	if (swiotlb_force == SWIOTLB_FORCE ||
+	    max_pfn > arm_dma_pfn_limit)
+		swiotlb_init(1);
+	else
+		swiotlb_force = SWIOTLB_NO_FORCE;
 #endif
 
 	set_max_mapnr(pfn_to_page(max_pfn) - mem_map);
-- 
2.25.1

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Christoph Hellwig]

On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
>  #ifdef CONFIG_ARM_LPAE
> +	if (swiotlb_force == SWIOTLB_FORCE ||
> +	    max_pfn > arm_dma_pfn_limit)

Does arm_dma_pfn_limit do the right thing even with the weirdest
remapping ranges?  Maybe a commen here would be useful.

> +		swiotlb_init(1);
> +	else
> +		swiotlb_force = SWIOTLB_NO_FORCE;

Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
That kind makes more sense than forcing the callers to do it.

While we're at it, I think swiotlb_force should probably be renamed to
swiotlb_mode or somethng like that.

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Florian Fainelli]

On 3/19/2021 6:07 AM, Christoph Hellwig wrote:
> On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
>>  #ifdef CONFIG_ARM_LPAE
>> +	if (swiotlb_force == SWIOTLB_FORCE ||
>> +	    max_pfn > arm_dma_pfn_limit)
> 
> Does arm_dma_pfn_limit do the right thing even with the weirdest
> remapping ranges?  Maybe a commen here would be useful.

It gets assigned to either 0xffffffff or PHYS_OFFSET + arm_dma_zone_size
- 1 which is obtained from the machine descriptor, so I expect it to do
the right thing, it works for a Pi 4 in 32-bit mode for instance. This
is conditional upon enabling CONFIG_ZONE_DMA for ARM, and will otherwise
keep its original value of 0, so this should be safe AFAICT.

> 
>> +		swiotlb_init(1);
>> +	else
>> +		swiotlb_force = SWIOTLB_NO_FORCE;
> 
> Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
> and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
> That kind makes more sense than forcing the callers to do it.
> 
> While we're at it, I think swiotlb_force should probably be renamed to
> swiotlb_mode or somethng like that.
Agreed.
-- 
Florian

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Konrad Rzeszutek Wilk]

On Fri, Mar 19, 2021 at 02:07:31PM +0100, Christoph Hellwig wrote:
> On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
> >  #ifdef CONFIG_ARM_LPAE
> > +	if (swiotlb_force == SWIOTLB_FORCE ||
> > +	    max_pfn > arm_dma_pfn_limit)
> 
> Does arm_dma_pfn_limit do the right thing even with the weirdest
> remapping ranges?  Maybe a commen here would be useful.
> 
> > +		swiotlb_init(1);
> > +	else
> > +		swiotlb_force = SWIOTLB_NO_FORCE;
> 
> Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
> and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
> That kind makes more sense than forcing the callers to do it.
> 
> While we're at it, I think swiotlb_force should probably be renamed to
> swiotlb_mode or somethng like that.

swiotlb_mode sounds good.

Also it got me thinking - ARM on Xen at some point was a bit strange, so not sure how
the logic works here, Stefano?

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Stefano Stabellini]

On Fri, 19 Mar 2021, Konrad Rzeszutek Wilk wrote:
> On Fri, Mar 19, 2021 at 02:07:31PM +0100, Christoph Hellwig wrote:
> > On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
> > >  #ifdef CONFIG_ARM_LPAE
> > > +	if (swiotlb_force == SWIOTLB_FORCE ||
> > > +	    max_pfn > arm_dma_pfn_limit)
> > 
> > Does arm_dma_pfn_limit do the right thing even with the weirdest
> > remapping ranges?  Maybe a commen here would be useful.
> > 
> > > +		swiotlb_init(1);
> > > +	else
> > > +		swiotlb_force = SWIOTLB_NO_FORCE;
> > 
> > Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
> > and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
> > That kind makes more sense than forcing the callers to do it.
> > 
> > While we're at it, I think swiotlb_force should probably be renamed to
> > swiotlb_mode or somethng like that.
> 
> swiotlb_mode sounds good.
> 
> Also it got me thinking - ARM on Xen at some point was a bit strange, so not sure how
> the logic works here, Stefano?

There is nothing strange in regards to swiotlb_force. swiotlb_force is only used
in swiotlb-xen map_page to figure out whether:

- we actually have to use the swiotlb bounce buffer (this is the
  swiotlb_xen == SWIOTLB_FORCE case)
- or we can use the provided page directly for dma if other conditions
  are met (dma_capable, !range_straddles_page_boundary, ...)


I don't think that switching to "swiotlb_mode" would cause any issues.

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Florian Fainelli]

On 3/19/21 5:22 PM, Stefano Stabellini wrote:
> On Fri, 19 Mar 2021, Konrad Rzeszutek Wilk wrote:
>> On Fri, Mar 19, 2021 at 02:07:31PM +0100, Christoph Hellwig wrote:
>>> On Thu, Mar 18, 2021 at 09:03:33PM -0700, Florian Fainelli wrote:
>>>>  #ifdef CONFIG_ARM_LPAE
>>>> +	if (swiotlb_force == SWIOTLB_FORCE ||
>>>> +	    max_pfn > arm_dma_pfn_limit)
>>>
>>> Does arm_dma_pfn_limit do the right thing even with the weirdest
>>> remapping ranges?  Maybe a commen here would be useful.
>>>
>>>> +		swiotlb_init(1);
>>>> +	else
>>>> +		swiotlb_force = SWIOTLB_NO_FORCE;
>>>
>>> Konrad: what do you think of setting swiotlb_force to SWIOTLB_NO_FORCE
>>> and only switching it to SWIOTLB_NORMAL when swiotlb_init* is called?
>>> That kind makes more sense than forcing the callers to do it.
>>>
>>> While we're at it, I think swiotlb_force should probably be renamed to
>>> swiotlb_mode or somethng like that.
>>
>> swiotlb_mode sounds good.
>>
>> Also it got me thinking - ARM on Xen at some point was a bit strange, so not sure how
>> the logic works here, Stefano?
> 
> There is nothing strange in regards to swiotlb_force. swiotlb_force is only used
> in swiotlb-xen map_page to figure out whether:
> 
> - we actually have to use the swiotlb bounce buffer (this is the
>   swiotlb_xen == SWIOTLB_FORCE case)
> - or we can use the provided page directly for dma if other conditions
>   are met (dma_capable, !range_straddles_page_boundary, ...)
> 
> 
> I don't think that switching to "swiotlb_mode" would cause any issues.
> 

Should I toss this in Russell's patch tracker or do you need me to make
some changes to the patch?
-- 
Florian

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Christoph Hellwig]

On Mon, Mar 29, 2021 at 12:30:42PM -0700, Florian Fainelli wrote:
> Should I toss this in Russell's patch tracker or do you need me to make
> some changes to the patch?

Due to all the other changes in this area I don't think anything but
the swiotlb tree makes much sense here.

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Konrad Rzeszutek Wilk]

On Tue, Mar 30, 2021 at 07:36:07AM +0200, Christoph Hellwig wrote:
> On Mon, Mar 29, 2021 at 12:30:42PM -0700, Florian Fainelli wrote:
> > Should I toss this in Russell's patch tracker or do you need me to make
> > some changes to the patch?
> 
> Due to all the other changes in this area I don't think anything but
> the swiotlb tree makes much sense here.

I've put them all on 

git://git.kernel.org/pub/scm/linux/kernel/git/konrad/swiotlb.git
devel/for-linus-5.13

Re: [PATCH] ARM: Qualify enabling of swiotlb_init()

[Florian Fainelli]

On 4/1/21 10:33 AM, Konrad Rzeszutek Wilk wrote:
> On Tue, Mar 30, 2021 at 07:36:07AM +0200, Christoph Hellwig wrote:
>> On Mon, Mar 29, 2021 at 12:30:42PM -0700, Florian Fainelli wrote:
>>> Should I toss this in Russell's patch tracker or do you need me to make
>>> some changes to the patch?
>>
>> Due to all the other changes in this area I don't think anything but
>> the swiotlb tree makes much sense here.
> 
> I've put them all on 
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/konrad/swiotlb.git
> devel/for-linus-5.13

Thanks! Did you also want to queue up this one:

https://lore.kernel.org/lkml/20210323015350.399493-1-f.fainelli@gmail.com/
-- 
Florian