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

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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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