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

["Sharma, Shashank" <shashank.sharma@intel.com>]

Regards
Shashank
> 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
None of these aspect ratios are new modes / new aspect ratios from HDMI 
2.0/CEA-861-F
These are the existing modes, and should be independent of reverted 
patches.
> 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.
I am writing a patch series to cap the aspect ratio implementation under 
a drm_cap_hdmi2_aspect_ratios
This is how its going to work (inspired from the 2D/stereo series from 
damien L)

- Add a new capability hdmi2_ar
- by default parsing the new hdmi 2.0 aspect ratio will be disabled 
under check of this cap
- during bootup time, while initializing the display, a userspace can 
get_cap on the hdmi2_aspect_ratio
- If it wants HDMI 2.0 aspect ratio support, it will set the cap, and 
kernel will expose these aspect ratios
> Another bug I think might be the ordering of the modes with aspect ratio
> specified. IIRC the spec says that the preferred aspect ratio should be
> listed first in the EDID, but I don't think we preserve that ordering
> in the final mode list. I guess we could fix that by somehow noting
> which aspect ratio is preferred and sort based on that, or we try to
> preserve the order from the EDID until we're ready to sort, and then do
> the sorting with a stable algorithm.
AFAIK The mode order and priority is decided and arranged in userspace, 
based on various factors like
- preferred mode.
- previously applied mode in previous sessions (like for android tvs)
- Bigger h/w vs better refresh rate ?
- Xserver applies its own algorithms to decide which mode should be 
shown first.

I dont think kernel needs to bother about it.

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