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

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Message-ID: <bc1ba8bd-4433-4f83-8ce6-73bed5abd843@quicinc.com>
Date: Sun, 27 Oct 2024 23:44:43 +0530
From: Akhil P Oommen <quic_akhilpo@...cinc.com>
To: Arnd Bergmann <arnd@...db.de>,
        Dmitry Baryshkov
	<dmitry.baryshkov@...aro.org>
CC: Arnd Bergmann <arnd@...nel.org>, Rob Clark <robdclark@...il.com>,
        "Abhinav
 Kumar" <quic_abhinavk@...cinc.com>,
        Dave Airlie <airlied@...il.com>, "Simona
 Vetter" <simona@...ll.ch>,
        Nathan Chancellor <nathan@...nel.org>, 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 10/21/2024 3:31 PM, Arnd Bergmann wrote:
> On Mon, Oct 21, 2024, at 09:25, 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.
> 
> Yes, that should work. I actually tried first to turn the model
> specific data into static const structures but that ended up 
> not working because some of them have a couple of dynamically
> computed values. I think that would have been even nicer.
> 
>       Arnd

https://patchwork.freedesktop.org/patch/621814/

Posted the fix. Btw, I have copied most of the commit text from this patch.

-Akhil.