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

[Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>]

On 28.10.2024 11:27 AM, Dmitry Baryshkov wrote:
> On Mon, 28 Oct 2024 at 12:08, Akhil P Oommen <quic_akhilpo@quicinc.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@kernel.org>
>>>>
>>>> Signed-off-by: Akhil P Oommen <quic_akhilpo@quicinc.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

Konrad