[PATCH] drm/msm/dpu: change _dpu_plane_calc_bw() to use u64 to avoid overflow

[Dmitry Baryshkov]

On 08/09/2023 03:52, Abhinav Kumar wrote:
> 
> 
> On 9/7/2023 5:35 PM, Dmitry Baryshkov wrote:
>> On 08/09/2023 03:32, Abhinav Kumar wrote:
>>> _dpu_plane_calc_bw() uses integer variables to calculate the bandwidth
>>> used during plane bandwidth calculations. However for high resolution
>>> displays this overflows easily and leads to below errors
>>
>> Can we move the u64 conversion closer to the place where we actually 
>> need it? Having u64 source width looks very strange.
>>
> 
> Its this math 
> https://gitlab.freedesktop.org/drm/msm/-/blob/msm-next/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c#L139 which overflows.
> 
> So its not just one variable but I can even change this to u32 as that 
> also fixes the issue. Let me know if u32 works better.
> 
> Perhaps I went too far to go from int to u64.

I'd prefer to have the u64 conversion around the actual calculations - 
so that we emphasise the issue, not the size of the width / etc.

> 
>>>
>>> [dpu error]crtc83 failed performance check -7
>>>
>>> Promote the intermediate variables to u64 to avoid overflow.
>>>
>>> Fixes: c33b7c0389e1 ("drm/msm/dpu: add support for clk and bw scaling 
>>> for display")
>>> Closes: https://gitlab.freedesktop.org/drm/msm/-/issues/32
>>> Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
>>> ---
>>>   drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c 
>>> b/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
>>> index ae970af1154f..c6193131beec 100644
>>> --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
>>> +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
>>> @@ -118,7 +118,7 @@ static u64 _dpu_plane_calc_bw(const struct 
>>> dpu_mdss_cfg *catalog,
>>>       const struct drm_display_mode *mode,
>>>       struct dpu_sw_pipe_cfg *pipe_cfg)
>>>   {
>>> -    int src_width, src_height, dst_height, fps;
>>> +    u64 src_width, src_height, dst_height, fps;
>>>       u64 plane_prefill_bw;
>>>       u64 plane_bw;
>>>       u32 hw_latency_lines;
>>

-- 
With best wishes
Dmitry