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

[Daniel Vetter]

On Thu, May 26, 2022 at 09:01:03AM -0700, Doug Anderson wrote:
> 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?

Sounds reasonable.
-Daniel

> 
> 
> > 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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch