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

[Abhinav Kumar]

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@xxxxxxxxxxx>
> > ---
> >   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;