Re: [RFC PATCH 3/4] drm/panfrost: Support ARM_64_LPAE_S1 page table
From: Ariel D'Alessandro
Date: Mon Mar 10 2025 - 11:46:29 EST
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@xxxxxxxxxxxxx>
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.
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