Re: [PATCH 1/1] drm: xlnx: zynqmp: Stop the loop at lowest link rate without check

[Hyun Kwon <hyun.kwon@xilinx.com>]

Hi Daniel,

Thanks for the review.

On Wed, Jul 29, 2020 at 02:34:16PM -0700, Daniel Vetter wrote:
> On Wed, Jul 29, 2020 at 8:21 PM Hyun Kwon <hyun.kwon@xilinx.com> wrote:
> >
> > The loop should exit at the lowest link rate, so break the loop
> > at the lowest link rate without check. The check is always true
> > because lowest link rate is smaller than current one and maximum
> > of current display. Otherwise, it may be seen as the loop can
> > potentially result in negative array offset.
> >
> > The patch d76271d22694: "drm: xlnx: DRM/KMS driver for Xilinx ZynqMP
> > DisplayPort Subsystem" from Jul 7, 2018, leads to the following
> > static checker warning:
> >
> >         drivers/gpu/drm/xlnx/zynqmp_dp.c:594 zynqmp_dp_mode_configure()
> >         error: iterator underflow 'bws' (-1)-2
> >
> > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
> > ---
> >  drivers/gpu/drm/xlnx/zynqmp_dp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/xlnx/zynqmp_dp.c b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > index b735072..1be2b19 100644
> > --- a/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > +++ b/drivers/gpu/drm/xlnx/zynqmp_dp.c
> > @@ -579,7 +579,7 @@ static int zynqmp_dp_mode_configure(struct zynqmp_dp *dp, int pclock,
> >                 return -EINVAL;
> >         }
> >
> > -       for (i = ARRAY_SIZE(bws) - 1; i >= 0; i--) {
> > +       for (i = ARRAY_SIZE(bws) - 1; i > 0; i--) {
> 
> But now we don't go through the lowest element anymore, which also
> looks wrong. Or I'm blind.
> 

Currently, the lowest element always breaks without decrement by the check of
the loop.

> I think the problem is later on that we should bail out of the loop on
> the last iteration (when i == 0) before we decrement, since otherwise
> we then look at bws[-1] in the next loop, which is clearly wrong. I
> guess your code results in the same, but it's very confusing logic for
> me ...

Indeed. I can convert the for loop into switch - case in v2. Hope it makes less
confusing. :)

Thanks,
-hyun

> -Daniel
> 
> >                 if (current_bw && bws[i] >= current_bw)
> >                         continue;
> >
> > --
> > 2.7.4
> >
> > _______________________________________________
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> 
> 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch
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