[PATCH] drm: Add DRM_MODE_TYPE_USERDEF flag to probed modes matching a video= argument

[Hans de Goede]

Hi,

On 11-11-2019 10:25, Daniel Vetter wrote:
> On Sun, Nov 10, 2019 at 7:41 PM Hans de Goede <hdegoede at redhat.com> wrote:
>>
>> drm_helper_probe_add_cmdline_mode() prefers using a probed mode matching
>> a video= argument over calculating our own timings for the user specified
>> mode using CVT or GTF.
>>
>> But userspace code which is auto-configuring the mode may want to know that
>> the user has specified that mode on the kernel commandline so that it can
>> pick that mode over the mode which is marked as DRM_MODE_TYPE_PREFERRED.
>>
>> This commit sets the DRM_MODE_TYPE_USERDEF flag on the matching mode, just
>> as we would do on the user-specified mode when no matching probed mode is
>> found.
>>
>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> 
> Will existing userspace dtrt here with this? Some links to popular
> ones would be good (since essentially this is uapi fine tuning we need
> that anyway). With that will get my ack.

A valid question, I've gone over what I consider the major userspace kms users:
-Xorg xserver modesetting driver does not check for this:
  [hans at shalem xserver]$ ack DRM_MODE_TYPE_ hw/xfree86/drivers/modesetting/
  hw/xfree86/drivers/modesetting/drmmode_display.c
  1321:    if (kmode->type & DRM_MODE_TYPE_DRIVER)
  1323:    if (kmode->type & DRM_MODE_TYPE_PREFERRED)
-Other Xorg drivers:
  [hans at shalem driver]$ ls -d xf86-video-*
  xf86-video-amdgpu  xf86-video-intel        xf86-video-qxl
  xf86-video-armsoc  xf86-video-modesetting  xf86-video-sisusb
  xf86-video-ati     xf86-video-nouveau      xf86-video-vmware
  xf86-video-dummy   xf86-video-omap         xf86-video-voodoo
  xf86-video-geode   xf86-video-opentegra
  These all only do the same checks as the Xorg modesetting driver
-mutter:
  [hans at shalem mutter]$ ack DRM_MODE_TYPE_
  src/backends/native/meta-output-kms.c
  261:      if (drm_mode->type & DRM_MODE_TYPE_PREFERRED)

So it seems nothing (that I can quickly find) in userspace is using this atm.

The reason I wrote this patch is because about a year ago plymouth used to
fully rely on the kernel to setup the modes on monitors and would simply
inherit the modes setup by the kernel. Basically plymouth was relying on
fbcon to load first and setup modes.

Deferred fbcon takeover (for flickerfree) means that this is no longer
happening. So now plymouth picks a mode itself. When I submitted the
plymouth change for plymouth to pick a mode itself the plymouth maintainer
(Ray Strode) was afraid that would break plymouth honoring video= arguments.
So currently plymouth still relies on the kernel to do the mode setup if
a video= argument is present on the kernel commandline.

My other recent series, which adds support for e.g.
video=HDMI:panel_orientation=right_side_up, made me realize that relying
on the kernel for this is no good since the fbcon code has various
limitations wrt e.g. hotplug and this of course will not work when
fbcon deferred takeover is used and fbcon never loads before plymouth.

So I wrote a patch for plymouth to check the DRM_MODE_TYPE_USERDEF
flag and prefer a mode with that flag over the PREFERRED mode:
https://gitlab.freedesktop.org/plymouth/plymouth/merge_requests/84

And then I found out that for that code to work with modes which
are already in the list of probed modes, I need something like
the kernel patch we are discussing now.

So if you want a link to an userspace consumer of this, I guess you
want a v2 with a link to the plymouth MR added. + maybe a blurb in
the commit message that to the best of my knowledge no userspace
kms consumers are checking for the DRM_MODE_TYPE_USERDEF flag?

Regards,

Hans






> -Danile
> 
>> ---
>>   drivers/gpu/drm/drm_probe_helper.c | 2 ++
>>   include/drm/drm_modes.h            | 3 ++-
>>   2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_probe_helper.c b/drivers/gpu/drm/drm_probe_helper.c
>> index ef2c468205a2..4fed64be11f9 100644
>> --- a/drivers/gpu/drm/drm_probe_helper.c
>> +++ b/drivers/gpu/drm/drm_probe_helper.c
>> @@ -157,6 +157,8 @@ static int drm_helper_probe_add_cmdline_mode(struct drm_connector *connector)
>>                                  continue;
>>                  }
>>
>> +               /* Mark the matching mode as being preferred by the user */
>> +               mode->type |= DRM_MODE_TYPE_USERDEF;
>>                  return 0;
>>          }
>>
>> diff --git a/include/drm/drm_modes.h b/include/drm/drm_modes.h
>> index e946e20c61d8..c7efb7487e9b 100644
>> --- a/include/drm/drm_modes.h
>> +++ b/include/drm/drm_modes.h
>> @@ -256,7 +256,8 @@ struct drm_display_mode {
>>           *  - DRM_MODE_TYPE_DRIVER: Mode created by the driver, which is all of
>>           *    them really. Drivers must set this bit for all modes they create
>>           *    and expose to userspace.
>> -        *  - DRM_MODE_TYPE_USERDEF: Mode defined via kernel command line
>> +        *  - DRM_MODE_TYPE_USERDEF: Mode defined or selected via the kernel
>> +        *    command line.
>>           *
>>           * Plus a big list of flags which shouldn't be used at all, but are
>>           * still around since these flags are also used in the userspace ABI.
>> --
>> 2.23.0
>>
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> 
> 
>