linux-kernel - Re: [PATCH] drm: a6xx: avoid excessive stack usage

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Message-ID: <c80309ad-52de-4998-ab0e-05db7cc5068b@oss.qualcomm.com>
Date: Mon, 21 Oct 2024 11:31:52 +0200
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Akhil P Oommen <quic_akhilpo@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Arnd Bergmann <arnd@...nel.org>, Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Nathan Chancellor <nathan@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Sean Paul <sean@...rly.run>, Konrad Dybcio <konradybcio@...nel.org>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Bill Wendling
 <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm: a6xx: avoid excessive stack usage

On 21.10.2024 11:25 AM, Akhil P Oommen wrote:
> On Sat, Oct 19, 2024 at 04:14:13PM +0300, Dmitry Baryshkov wrote:
>> On Sat, Oct 19, 2024 at 03:01:46PM +0530, Akhil P Oommen wrote:
>>> On Fri, Oct 18, 2024 at 03:11:38PM +0000, Arnd Bergmann wrote:
>>>> From: Arnd Bergmann <arnd@...db.de>
>>>>
>>>> Clang-19 and above sometimes end up with multiple copies of the large
>>>> a6xx_hfi_msg_bw_table structure on the stack. The problem is that
>>>> a6xx_hfi_send_bw_table() calls a number of device specific functions to
>>>> fill the structure, but these create another copy of the structure on
>>>> the stack which gets copied to the first.
>>>>
>>>> If the functions get inlined, that busts the warning limit:
>>>>
>>>> drivers/gpu/drm/msm/adreno/a6xx_hfi.c:631:12: error: stack frame size (1032) exceeds limit (1024) in 'a6xx_hfi_send_bw_table' [-Werror,-Wframe-larger-than]
>>>
>>> Why does this warning says that the limit is 1024? 1024 bytes is too small, isn't it?
>>
>> Kernel stacks are expected to be space limited, so 1024 is a logical
>> limit for a single function.
> 
> Thanks for the clarification. I think it is better to move this table to
> struct a6xx_gmu which is required anyway when we implement dynamic generation
> of bw table. Also, we can skip initializing it in subsequent gpu wake ups.
> 
> Arnd, do you think that would be sufficient? I can send that patch if you
> want help.

FWIW I implemented this at one point.. ended up only rebasing it for months
as I kept delaying GMU voting until we get an in-tree dram frequency LUT
retrieving driver..

https://github.com/SoMainline/linux/commits/konrad/longbois-next/drivers/gpu/drm/msm/adreno

worth noting that this used to be my R&D branch so this is very much err..
"provided as-is".. but it did work on 8250!

Konrad