Re: [PATCH 1/2] Revert "drm: Add and handle new aspect ratios in DRM layer"

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

On Fri, Nov 11, 2016 at 07:00:17PM +0200, Ville Syrjälä wrote:
> On Thu, Nov 03, 2016 at 02:31:43PM +0200, ville.syrjala@linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > This reverts commit a68362fe3e84fcbedd49939aa200519aa5410135.
> > 
> > Adding new mode flags willy nilly breaks existing userspace. We need to
> > coordinate this better, potentially with a new client cap that only
> > exposes the aspect ratio flags when userspace is prepared for them
> > (similar to what we do with stereo 3D modes).
> 
> As a demonstration here's the change in the xrandr mode list after doing
> the revert:
> 
>  HDMI2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 700mm x 390mm
> -   1920x1080     60.00*+
> -   1920x1080i    60.00    50.00  
> +   1920x1080     60.00*+  50.00    59.94    30.00    25.00    24.00    29.97    23.98  
> +   1920x1080i    60.00    50.00    59.94  
>     1600x1200     60.00  
>     1680x1050     59.88  
>     1280x1024     75.02    60.02  
> @@ -13,30 +13,29 @@
>     1360x768      60.02  
>     1280x800      59.91  
>     1152x864      75.00  
> -   1280x720      60.00    50.00  
> +   1280x720      60.00    50.00    59.94  
>     1024x768      75.03    70.07    60.00  
>     832x624       74.55  
>     800x600       72.19    75.00    60.32  
> -   640x480       75.00    72.81    66.67    59.94  
> +   720x576       50.00  
> +   720x480       60.00    59.94  
> +   640x480       75.00    72.81    66.67    60.00    59.94  
>     720x400       70.08  
> 
> This was with sna, which does this:
>  #define KNOWN_MODE_FLAGS ((1<<14)-1)
>  if (mode->status == MODE_OK && kmode->flags & ~KNOWN_MODE_FLAGS)
>  	mode->status = MODE_BAD; /* unknown flags => unhandled */
> so all the modes with an aspect ratio just vanished.
> 
> -modesetting and -ati on the other hand just copy over the unknown
> bits into the xrandr mode structure, which sounds dubious at best:
>  mode->Flags = kmode->flags; //& FLAG_BITS;
> I've not checked what damage it can actually cause.
> 
> 
> It looks like a few modes disappeared from the kernel's mode list
> as well, presumably because some cea modes in the list originated from
> DTDs and whanot so they don't have an aspect ratio and that causes 
> add_alternate_cea_modes() to ignore them. So not populating an aspect
> ratio for cea modes originating from a source other than 
> edid_cea_modes[] looks like another bug to me as well.

Oh and I guess this is also the reason most people didn't notice
anything wrong. The preferred mode usually (or maybe always?) comes from
some other source than edid_cea_modes[] and hence doesn't tend to
go AWOL.

-- 
Ville Syrjälä
Intel OTC
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