linux-kernel - Re: [PATCH 0/2] drm/panthor: add custom ASN hash support

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Message-ID: <ac4838eb-7613-4642-a007-577a9f665984@arm.com>
Date: Fri, 29 Aug 2025 14:40:58 +0100
From: Steven Price <steven.price@....com>
To: Chia-I Wu <olvaffe@...il.com>,
 Boris Brezillon <boris.brezillon@...labora.com>,
 Liviu Dudau <liviu.dudau@....com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, dri-devel@...ts.freedesktop.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] drm/panthor: add custom ASN hash support

On 28/08/2025 21:18, Chia-I Wu wrote:
> Some socs such as mt8196 require custom ASN hash.

I don't know the full details of this, but I'm puzzled by the "require"
here.

AIUI the "custom ASN hash support" (or L2C_SLICE_HASH as it was renamed)
affects the efficiency of the L2 caches in the GPU. It basically
determines how addresses are striped over the individual caches.

So (unless there is a specific errata) not setting a custom hash will
work just fine, but the performance might be slightly reduced.

kbase provides both a DT option and a module parameter which can be used
to override the defaults (although the parameter descriptions say "for
testing" which I think is somewhat telling).

How we should describe this somewhat depends on whether this is a
specific workaround for a SoC - in which case Boris's suggestion of
using a different compatible string and panthor_soc_data seems like a
good choice. Or if this is exposed as a general "tuning" parameter, in
which case this might be appropriate.

I believe the tuning is related to more than just a SoC (the external
memory system has an impact). So I guess a DT level knob makes most
sense here.

Steve

> Chia-I Wu (2):
>   dt-bindings: gpu: mali-valhall-csf: add asn-hash
>   drm/panthor: add asn-hash support
> 
>  .../bindings/gpu/arm,mali-valhall-csf.yaml    |  8 ++++++
>  drivers/gpu/drm/panthor/panthor_device.c      | 28 +++++++++++++++++++
>  drivers/gpu/drm/panthor/panthor_device.h      |  6 ++++
>  drivers/gpu/drm/panthor/panthor_gpu.c         | 17 +++++++++++
>  drivers/gpu/drm/panthor/panthor_regs.h        |  4 +++
>  5 files changed, 63 insertions(+)
>