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

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Message-ID: <vtj3yahojkozge4bvq66ax2c2idbw27c3hs7l6cy3e7ucz4jqb@qge2nckj3mr4>
Date: Mon, 28 Oct 2024 12:52:01 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
Cc: 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>, 
	Marijn Suijten <marijn.suijten@...ainline.org>, David 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>, linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH] drm/msm/a6xx: Fix excessive stack usage

On Mon, Oct 28, 2024 at 11:36:15AM +0100, Konrad Dybcio wrote:
> On 28.10.2024 11:27 AM, Dmitry Baryshkov wrote:
> > On Mon, 28 Oct 2024 at 12:08, Akhil P Oommen <quic_akhilpo@...cinc.com> wrote:
> >>
> >> On 10/28/2024 1:56 PM, Dmitry Baryshkov wrote:
> >>> On Sun, Oct 27, 2024 at 11:35:47PM +0530, 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>
> >>>>
> >>>> Signed-off-by: Akhil P Oommen <quic_akhilpo@...cinc.com>
> >>>> ---
> >>>>  drivers/gpu/drm/msm/adreno/a6xx_gmu.h |  1 +
> >>>>  drivers/gpu/drm/msm/adreno/a6xx_hfi.c | 34 ++++++++++++++++++++++------------
> >>>>  2 files changed, 23 insertions(+), 12 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gmu.h 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;
> >>>>  };
> >>>>
> >>>>  static inline u32 gmu_read(struct a6xx_gmu *gmu, u32 offset)
> >>>> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_hfi.c b/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>> index cdb3f6e74d3e..55e51c81be1f 100644
> >>>> --- a/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>> +++ b/drivers/gpu/drm/msm/adreno/a6xx_hfi.c
> >>>> @@ -630,32 +630,42 @@ static void a6xx_build_bw_table(struct a6xx_hfi_msg_bw_table *msg)
> >>>>
> >>>>  static int a6xx_hfi_send_bw_table(struct a6xx_gmu *gmu)
> >>>>  {
> >>>> -    struct a6xx_hfi_msg_bw_table msg = { 0 };
> >>>> +    struct a6xx_hfi_msg_bw_table *msg;
> >>>>      struct a6xx_gpu *a6xx_gpu = container_of(gmu, struct a6xx_gpu, gmu);
> >>>>      struct adreno_gpu *adreno_gpu = &a6xx_gpu->base;
> >>>>
> >>>> +    if (gmu->bw_table)
> >>>> +            goto send;
> >>>> +
> >>>> +    msg = devm_kzalloc(gmu->dev, sizeof(*msg), GFP_KERNEL);
> >>>
> >>> Is it necessary after being sent? Isn't it better to just kzalloc() it
> >>> and then kfree() it at the end of the function?
> >>
> >> Keeping it around will help to cut down unnecessary work during
> >> subsequent gpu wake ups.
> > 
> > Then, I'd say, it is better to make it a part of the a6xx_gpu struct.
> 
> I think a6xx_gmu makes more logical sense here.
> 
> FWIW, the driver allocates both _gmu and _gpu for all GPUs regardless

Hmm, are we expected to handle / perform BW requests in case of GMU-less
devices?

-- 
With best wishes
Dmitry