Date:   Wed, 13 Dec 2023 16:34:03 +0100
From:   Maxime Ripard <mripard@...nel.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Pin-yen Lin <treapking@...omium.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Jessica Zhang <quic_jesszhan@...cinc.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, linux-kernel@...r.kernel.org,
        Guenter Roeck <groeck@...omium.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 4/4] drm/panel-edp: Add some panels with conservative
 timings

On Thu, Dec 07, 2023 at 11:14:34AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Thu, Dec 7, 2023 at 10:58 AM Maxime Ripard <mripard@...nel.org> wrote:
> >
> > On Thu, Dec 07, 2023 at 10:23:53AM -0800, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Thu, Dec 7, 2023 at 12:18 AM Pin-yen Lin <treapking@...omium.org> wrote:
> > > >
> > > > These panels are used by Mediatek MT8173 Chromebooks but we can't find
> > > > the corresponding data sheets, and these panels used to work on v4.19
> > > > kernel without any specified delays.
> > > >
> > > > Therefore, instead of having them use the default conservative timings,
> > > > update them with less-conservative timings from other panels of the same
> > > > vendor. The panels should still work under those timings, and we can
> > > > save some delays and suppress the warnings.
> > > >
> > > > Signed-off-by: Pin-yen Lin <treapking@...omium.org>
> > > >
> > > > ---
> > > >
> > > > (no changes since v1)
> > > >
> > > >  drivers/gpu/drm/panel/panel-edp.c | 31 +++++++++++++++++++++++++++++++
> > > >  1 file changed, 31 insertions(+)
> > >
> > > Reviewed-by: Douglas Anderson <dianders@...omium.org>
> > >
> > > Repeating my comments from v1 here too, since I expect this patch to
> > > sit on the lists for a little while:
> > >
> > >
> > > This is OK w/ me, but it will need time on the mailing lists before
> > > landing in case anyone else has opinions.
> >
> > Generally speaking, I'm not really a fan of big patches that dump
> > whatever ChromeOS is doing ...
> >
> > > Specifical thoughts:
> > >
> > > * I at least feel fairly confident that this is OK since these panels
> > > essentially booted without _any_ delays back on the old downstream
> > > v4.19 kernel. Presumably the panels just had fairly robust timing
> > > controllers and so worked OK, but it's better to get the timing more
> > > correct.
> >
> > ... especially since you have to rely on the recollection of engineers
> > involved at the time and you have no real way to test and make things
> > clearer anymore, and we have to take patches in that are handwavy "trust
> > us, it's doing the right thing".
> >
> > I'd really prefer to have these patches sent as they are found out.
> 
> It's probably not clear enough from the commit message, but this isn't
> just a dump from downstream 4.19. What happened was:
> 
> 1. Downstream chromeos-4.19 used the "little white lie" approach. They
> all claimed a specific panel's compatible string even though there
> were a whole pile of panels out there actually being used. Personally,
> this was not something I was ever a fan of (and I wasn't personally
> involved in this project), but it was the "state of the art" before
> the generic panel-edp. Getting out of the "little white lie" situation
> was why I spent so much time on the generic edp-panel solution
> upstream.
> 
> 2. These devices have now been uprevved to a newer kernel and I
> believe that there were issues seen that necessitated a move to the
> proper generic panel-edp code.
> 
> 3. We are now getting field reports from our warning collector about a
> whole pile of panels that are falling back to the "conservative"
> timings, which means that they turn on/off much more slowly than they
> should.
> 
> Pin-yen made an attempt to search for panels data sheets that matched
> up with the IDs that came in from the field reports but there were
> some panels that he just couldn't find.
> 
> So basically we're stuck. Options:
> 
> 1. Leave customers who have these panels stuck with the hardware
> behaving worse than it used to. This is not acceptable to me.
> 
> 2. Land Pin-yen's patch as a downstream-only patch in ChromeOS. This
> would solve the problem, but it would make me sad. If anyone ever
> wants to take these old laptops and run some other Linux distribution
> on them (and there are several that target old Chromebooks) then
> they'd be again stuck with old timings.
> 
> 3. Land a patch like this one that at least gets us into not such a bad shape.
> 
> While I don't love this patch (and that's why I made it clear that it
> needs to spend time on the list), it seems better than the
> alternatives. Do you have a proposal for something else? If not, can
> you confirm you're OK with #3 given this explanation? ...and perhaps
> more details in the commit message?

I don't have a specific comment, it was more of a comment about the
process itself, if you write down what's above in the commit message ...

> I would also note that, hopefully, patches like this shouldn't be a
> recurring pattern. Any new Chromebooks using panel-edp will get
> flagged much earlier and we should be able to get real/proper timings
> added. I believe that we've also added a factory test so that
> (assuming it doesn't get ignored by someone) devices that aren't
> supported don't even make it out of the factory.

... and if we can expect it to be a one-off, then it's fine for me.

Thanks!
Maxime

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