Re: [Intel-gfx] [PATCH] drm/i915/display: Return correct err code for bpc < 0

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Mon, Apr 17, 2023 at 03:48:12PM -0700, Manasi Navare wrote:
> Hi Ville,
> 
> Could you suggest how to handle the intel_dp_link_compute_config()
> when the max_bpp is returned as 0, currently
> it just exits the loop and returns a -EINVAL and this triggers the DSC path.
> While we should be completely failing the modeset and encoder_config
> in this case instead of trying DSC, correct?

The DSC path needs to handle the bpp limits correctly:
1. Take the baseline limits already computed
2. Further restrict them based on sink/source DSC capabilities/etc.
3. Make sure the uncompressed bpp value chosen is between the min/max

> 
> Manasi
> 
> On Thu, Apr 13, 2023 at 8:23 AM Manasi Navare <navaremanasi@chromium.org> wrote:
> >
> > On Tue, Apr 11, 2023 at 10:22 PM Ville Syrjälä
> > <ville.syrjala@linux.intel.com> wrote:
> > >
> > > On Tue, Apr 11, 2023 at 05:07:01PM -0700, Manasi Navare wrote:
> > > > On Tue, Apr 11, 2023 at 10:42 AM Ville Syrjälä
> > > > <ville.syrjala@linux.intel.com> wrote:
> > > > >
> > > > > On Tue, Apr 11, 2023 at 05:34:08PM +0000, Manasi Navare wrote:
> > > > > > In the function intel_dp_max_bpp(), currently if bpc < 0 in case of error,
> > > > > > we return 0 instead of returning an err code of -EINVAL.
> > > > > > This throws off the logic in the calling function.
> > > > >
> > > > > What logic? The caller doesn't expect to get an error.
> > > >
> > > > If this returns a 0, we end up using limits.max_bpp = 0 and in
> > > > intel_dp_compute_link_config_wide(),
> > > > since max_bpp is 0, it exits this for loop:
> > > >
> > > > for (bpp = limits->max_bpp; bpp >= limits->min_bpp; bpp -= 2 * 3) and returns
> > > > -EINVAL which then wrongly goes to enable DSC even when link BW is
> > > > sufficient without DSC.
> > >
> > > And how woud max_bpp<0 prevent that?
> > >
> > > The real problem seems to be that the DSC code totally
> > > ignores bpp limits.
> >
> > Hi Ville,
> >
> > So I see a few concerns/questions:
> > - Why is the Max bpp value 0 in intel_dp_max_bpp, is that a valid case
> > and how should our link configurations handle that case when max_bpp
> > is 0?
> > - This is happening in a bug I am looking at with HDMI PCON, @Ankit
> > Nautiyal  have we ever seen something similar where max_bpp for HDMi
> > PCON
> > is returned 0?
> > - I dont think its a problem with DSC code, but rather
> > intel_dp_compute_link_config() outer for loop where we vary
> > from max_bpp to min_bpp and see if any bpp works with available link
> > bw, how should we handle this when max_bpp = 0 if you are saying thats
> > a valid case?
> > - In this patch if I return -EINVAL instead of 0, then atleast the
> > entire encoder_config will fail and that will fail the modeset, since
> > it assumes max_bpp cannot be 0
> >
> > Could you please help answer above concerns and how to handle max bpp
> > = 0 case if that is valid? This patch is simply making that invalid
> > resulting into modeset failure
> >
> > Manasi
> > >
> > > --
> > > Ville Syrjälä
> > > Intel

-- 
Ville Syrjälä
Intel