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

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Message-ID: <27bea27a-9aa3-42f5-9b0a-df5744276966@oss.qualcomm.com>
Date: Mon, 28 Oct 2024 12:31:50 +0100
From: Konrad Dybcio <konrad.dybcio@....qualcomm.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        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 28.10.2024 11:52 AM, Dmitry Baryshkov wrote:
> 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?

opp-table does that for us

In case of no gmu ("gmu wrapper"), Linux is the only entity that controls
things

Konrad