Re: [RFC PATCH 3/4] drm/panfrost: Support ARM_64_LPAE_S1 page table

[Ariel D'Alessandro <ariel.dalessandro@collabora.com>]

Hi Steven,

On 2/27/25 11:44 AM, Steven Price wrote:
> On 26/02/2025 18:30, Ariel D'Alessandro wrote:
>> Bifrost MMUs support AArch64 4kB granule specification. However,
>> panfrost only enables MMU in legacy mode, despite the presence of the
>> HW_FEATURE_AARCH64_MMU feature flag.
>>
>> This commit adds support to use page tables according to AArch64 4kB
>> granule specification. This feature is enabled conditionally based on
>> the GPU model's HW_FEATURE_AARCH64_MMU feature flag.
>>
>> Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
> 
> I find some of the naming confusing here. The subject calls it
> 'ARM_64_LPAE_S1' which in an unfortunate name from the iommu code.
> 
> AIUI, LPAE is the "Large Physical Address Extension" and is a v7 feature
> for 32 bit. "LEGACY" (as Bifrost calls it) mode is a (modified) version
> of LPAE, which in Linux we've called "mali_lpae".
> 
> What you're adding support for is AARCH64_4K which is the v8 64 bit
> mode. So I think it's worth including the "64" part of the name of the
> mmu_lpae_s1_enable() function. Personally I'd be tempted to drop the
> "_s1" part, but I guess there's a small chance someone will find a use
> for the second stage one day.

Yes, overall agreed. I'll definitely keep the "64_4k" part of the name 
explicit and also keep the "mali_lpae" for the legacy mode. Will send a 
patchset v1 soon with all this, we can continue reviewing it there.

> 
> Note also that it's not necessarily a clear-cut improvement to use
> AARCH64_4K over LEGACY. I wouldn't be surprised if this actually causes
> (minor) performance regressions on some platforms. Sadly I don't have
> access to a range of hardware to test this on.

FWIW, I'm using Mesa CI [0] to test this as much as possible, at least 
to detect any (major) regressions. As proposed by Boris, we should use a 
property to progressively enable this feature.

Thanks!

[0] https://gitlab.freedesktop.org/mesa/mesa/

-- 
Ariel D'Alessandro
Software Engineer

Collabora Ltd.
Platinum Building, St John's Innovation Park, Cambridge CB4 0DS, UK 
Registered in England & Wales, no. 5513718