linux-kernel - Re: [PATCH] drm/msm/a6xx: Fix excessive stack usage

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Message-Id: <3fb376b3-2db7-4730-a2e1-958f1ddd9f5c@app.fastmail.com>
Date: Sun, 27 Oct 2024 18:43:52 +0000
From: "Arnd Bergmann" <arnd@...nel.org>
To: "Akhil P Oommen" <quic_akhilpo@...cinc.com>,
 "Rob Clark" <robdclark@...il.com>, "Sean Paul" <sean@...rly.run>,
 "Konrad Dybcio" <konradybcio@...nel.org>,
 "Abhinav Kumar" <quic_abhinavk@...cinc.com>,
 "Dmitry Baryshkov" <dmitry.baryshkov@...aro.org>,
 "Marijn Suijten" <marijn.suijten@...ainline.org>,
 "Dave Airlie" <airlied@...il.com>, "Simona Vetter" <simona@...ll.ch>,
 "Nathan Chancellor" <nathan@...nel.org>,
 "Nick Desaulniers" <ndesaulniers@...gle.com>,
 "Bill Wendling" <morbo@...gle.com>, "Justin Stitt" <justinstitt@...gle.com>
Cc: 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/msm/a6xx: Fix excessive stack usage

On Sun, Oct 27, 2024, at 18:05, Akhil P Oommen wrote:
> 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]
>
> Fix this by kmalloc-ating struct a6xx_hfi_msg_bw_table instead of using
> the stack. Also, use this opportunity to skip re-initializing this table
> to optimize gpu wake up latency.
>
> Cc: Arnd Bergmann <arnd@...nel.org>

Please change this to "Reported-by:"

The patch looks correct to me, just one idea for improvement.

> b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> index 94b6c5cab6f4..b4a79f88ccf4 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gmu.h
> @@ -99,6 +99,7 @@ struct a6xx_gmu {
>  	struct completion pd_gate;
> 
>  	struct qmp *qmp;
> +	struct a6xx_hfi_msg_bw_table *bw_table;
>  };

I think the bw_table is better just embedded
in here rather than referenced as a pointer:

> +	if (gmu->bw_table)
> +		goto send;
> +
> +	msg = devm_kzalloc(gmu->dev, sizeof(*msg), GFP_KERNEL);
> +	if (!msg)
> +		return -ENOMEM;

It looked like it's always allocated here when the device
is up, so you can avoid the extra overhead for keeping
track of the allocation.

      Arnd