linux-kernel - Re: [PATCH 4/5] drm/modes: Add support for driver-specific named modes

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Date:   Mon, 11 Jul 2022 14:02:43 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Thomas Zimmermann <tzimmermann@...e.de>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans de Goede <hdegoede@...hat.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux/m68k <linux-m68k@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] drm/modes: Add support for driver-specific named
 modes

On Mon, Jul 11, 2022 at 01:59:28PM +0200, Geert Uytterhoeven wrote:
> Hi Maxime,
> 
> On Mon, Jul 11, 2022 at 1:42 PM Maxime Ripard <maxime@...no.tech> wrote:
> > On Mon, Jul 11, 2022 at 01:11:14PM +0200, Thomas Zimmermann wrote:
> > > Am 11.07.22 um 11:35 schrieb Maxime Ripard:
> > > > On Mon, Jul 11, 2022 at 11:03:38AM +0200, Thomas Zimmermann wrote:
> > > > > Am 08.07.22 um 20:21 schrieb Geert Uytterhoeven:
> > > > > > The mode parsing code recognizes named modes only if they are explicitly
> > > > > > listed in the internal whitelist, which is currently limited to "NTSC"
> > > > > > and "PAL".
> > > > > >
> > > > > > Provide a mechanism for drivers to override this list to support custom
> > > > > > mode names.
> > > > > >
> > > > > > Ideally, this list should just come from the driver's actual list of
> > > > > > modes, but connector->probed_modes is not yet populated at the time of
> > > > > > parsing.
> > > > >
> > > > > I've looked for code that uses these names, couldn't find any. How is this
> > > > > being used in practice? For example, if I say "PAL" on the command line, is
> > > > > there DRM code that fills in the PAL mode parameters?
> > > >
> > > > We have some code to deal with this in sun4i:
> > > > https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/sun4i/sun4i_tv.c#L292
> > > >
> > > > It's a bit off topic, but for TV standards, I'm still not sure what the
> > > > best course of action is. There's several interactions that make this a
> > > > bit troublesome:
> > > >
> > > >    * Some TV standards differ by their mode (ie, PAL vs NSTC), but some
> > > >      other differ by parameters that are not part of drm_display_mode
> > > >      (NTSC vs NSTC-J where the only difference is the black and blanking
> > > >      signal levels for example).
> > > >
> > > >    * The mode names allow to provide a fairly convenient way to add that
> > > >      extra information, but the userspace is free to create its own mode
> > > >      and might omit the mode name entirely.
> > > >
> > > > So in the code above, if the name has been preserved we match by name,
> > > > but we fall back to matching by mode if it hasn't been, which in this
> > > > case means that we have no way to differentiate between NTSC, NTSC-J,
> > > > PAL-M in this case.
> > > >
> > > > We have some patches downstream for the RaspberryPi that has the TV
> > > > standard as a property. There's a few extra logic required for the
> > > > userspace (like setting the PAL property, with the NTSC mode) so I'm not
> > > > sure it's preferable.
> > > >
> > > > Or we could do something like a property to try that standard, and
> > > > another that reports the one we actually chose.
> > > >
> > > > > And another question I have is whether this whitelist belongs into the
> > > > > driver at all. Standard modes exist independent from drivers or hardware.
> > > > > Shouldn't there simply be a global list of all possible mode names? Drivers
> > > > > would filter out the unsupported modes anyway.
> > > >
> > > > We should totally do something like that, yeah
> > >
> > > That sun code already looks like sometihng the DRM core/helpers should be
> > > doing. And if we want to support named modes well, there's a long list of
> > > modes in Wikipedia.
> > >
> > > https://en.wikipedia.org/wiki/Video_Graphics_Array#/media/File:Vector_Video_Standards2.svg
> >
> > Yeah, and NTSC is missing :)
> 
> And that diagram is about the "digital" variant of PAL.
> If you go the analog route, the only fixed parts are vfreq/hfreq,
> number of lines, and synchronization. Other parameters like overscan
> can vary.  The actual dot clock can vary wildly: while there is an
> upper limit due to bandwidth limitations, you can come up with an
> almost infinite number of video modes that can be called PAL, which
> is one of the reasons why I don't want hardware-specific variants to
> end up in a global video mode database.

Do you have an example of what that would look like?

Maxime

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