Re: [PATCH v3] drm/probe-helper: Make 640x480 first if no EDID

[Doug Anderson]

Hi,

On Thu, May 26, 2022 at 8:42 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
>
> On Thu, 26 May 2022 at 03:28, Sean Paul <seanpaul@xxxxxxxxxxxx> wrote:
> >
> > On Wed, May 25, 2022 at 9:26 AM Daniel Vetter <daniel@xxxxxxxx> wrote:
> > >
> > > On Mon, May 23, 2022 at 05:59:02PM -0700, Doug Anderson wrote:
> > > > Hi,
> > > >
> > > > On Fri, May 20, 2022 at 5:01 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > Hi,
> > > > >
> > > > > On Mon, May 16, 2022 at 3:28 AM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> > > > > >
> > > > > > Hi Douglas,
> > > > > >
> > > > > > I understand that you're trying to tell userspace that the modelist has
> > > > > > been made up, but it's not something that should be done via fragile
> > > > > > heuristics IMHO.
> > > > > >
> > > > > > I looked at the Chromium source code that you linked, but I cannot say
> > > > > > whether it's doing the correct thing. It all depends on what your
> > > > > > program needs.
> > > > > >
> > > > > > In that function, you could also search for 'DRM_MODE_TYPE_USERDEF'.
> > > > > > It's the mode that the user specified on the kernel command line. If
> > > > > > Chromium's automatic mode selection fails, you'd give your users direct
> > > > > > control over it.
> > > > >
> > > > > That doesn't really work for Chrome OS. Certainly a kernel hacker
> > > > > could do this, but it's not something I could imagine us exposing to
> > > > > an average user of a Chromebook.
> > > > >
> > > > >
> > > > > > When there's no flagged mode or if
> > > > > > /sys/class/drm/card<...>/status contains "unconnected", you can assume
> > > > > > that the modelist is artificial and try the modes in an appropriate order.
> > > > >
> > > > > So "no flagged" means that nothing is marked as preferred, correct?
> > > > >
> > > > > ...so I guess what you're suggesting is that the order that the kernel
> > > > > is presenting the modes to userspace is not ABI. If there are no
> > > > > preferred modes then userspace shouldn't necessarily assume that the
> > > > > first mode returned is the best mode. Instead it should assume that if
> > > > > there is no preferred mode then the mode list is made up and it should
> > > > > make its own decisions about the best mode to start with. If this is
> > > > > the ABI from the kernel then plausibly I could convince people to
> > > > > change userspace to pick 640x480 first in this case.
> > > > >
> > > > > > If we really want the kernel to give additional guarantees, we should
> > > > > > have a broader discussion about this topic IMHO.
> > > > >
> > > > > Sure. I've added Stéphane Marchesin to this thread in case he wants to
> > > > > chime in about anything.
> > > > >
> > > > > Overall, my take on the matter:
> > > > >
> > > > > * Mostly I got involved because, apparently, a DP compliance test was
> > > > > failing. The compliance test was upset that when it presented us with
> > > > > no EDID that we didn't default to 640x480. There was a push to make a
> > > > > fix for this in the Qualcomm specific driver but that didn't sit right
> > > > > with me.
> > > > >
> > > > > * On all devices I'm currently working with (laptops), the DP is a
> > > > > secondary display. If a user was trying to plug in a display with a
> > > > > bad EDID and the max mode (1024x768) didn't work, they could just use
> > > > > the primary display to choose a different resolution. It seems
> > > > > unlikely a user would truly be upset and would probably be happy they
> > > > > could get their broken display to work at all. Even if this is a
> > > > > primary display, I believe there are documented key combos to change
> > > > > the resolution of the primary display even if you can't see anything.
> > > > >
> > > > > * That all being said, defaulting to 640x480 when there's no EDID made
> > > > > sense to me, especially since it's actually defined in the DP spec. So
> > > > > I'm trying to do the right thing and solve this corner case. That
> > > > > being said, if it's truly controversial I can just drop it.
> > > > >
> > > > >
> > > > > So I guess my plan will be to give Stéphane a little while in case he
> > > > > wants to chime in. If not then I guess I'll try a Chrome patch...
> > > > > ...and if that doesn't work, I'll just drop it.
> > > >
> > > > OK, this userspace code seems to work:
> > > >
> > > > https://crrev.com/c/3662501 - ozone/drm: Try 640x480 before picking
> > > > the first mode if no EDID
> > > >
> > > > ...so we'll see how review of that goes. :-)
> >
> > Mirroring some of my comments on that review here :-)
> >
> > IMO, this should be addressed in the kernel, or not at all. The kernel
> > ensures other aspects of DisplayPort implementation are compliant, so
> > I don't think this would be any exception. Further, the kernel is the
> > one creating the "safe" mode list, so it seems odd that userspace
> > would override that. Finally, relying on every userspace to do the
> > right thing is asking for trouble (we have 3 places which would need
> > this logic in CrOS).
>
> Oh I missed the part that this is defined in the DP spec as _the_ fallback mode.
>
> I think the probe helpers could check whether it's a DP connector and
> then dtrt per DP spec? I think that should have a solid chance of
> avoiding the regression mess, since the really shoddy stuff tends to
> be VGA/HDMI.

I'm fine with making this DP-specific if that's what people think is best.


> Also if DP says only 640x480 should be the fallback if there's no
> other mode list source, then I think we should trim it down to only
> that. But also only for DP.

So the DP spec says that 640x480 is _the_ default fallback, but it
also says that we're also allowed to have some implementation-specific
fall-back modes as well, so I'd rather not fully trim the list and
just make it clear (somehow) that 640x480 ought to be the default.
Would you be OK going back to v2 of this patch [1] but adding a check
that the connector type is DP and also making sure that the spec is
referenced?


> Also ofc that patch should reference the right DP spec sections :-)

My original patch description for this patch (v3) did reference
section 4.2.2.6 (EDID Corruption Detection) of the DP 1.4a Link CTS.
...or did you want this in inline comments in the patch itself?


[1] https://lore.kernel.org/r/20220510135101.v2.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid