linux-kernel - Re: [PATCH] drm/msm/dpu: change _dpu_plane_calc

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Message-ID: <19aa1b04-e136-2e19-785d-a7856f4e8e41@quicinc.com>
Date:   Thu, 7 Sep 2023 17:52:44 -0700
From:   Abhinav Kumar <quic_abhinavk@...cinc.com>
To:     Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        <freedreno@...ts.freedesktop.org>, Rob Clark <robdclark@...il.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        "David Airlie" <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Kalyan Thota <quic_kalyant@...cinc.com>
CC:     <dri-devel@...ts.freedesktop.org>, <quic_jesszhan@...cinc.com>,
        <quic_parellan@...cinc.com>, <nespera@...lia.com>,
        Rob Clark <robdclark@...omium.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm/dpu: change _dpu_plane_calc_bw() to use u64 to
 avoid overflow



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.

>>
>> [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@...cinc.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;
>