Re: [PATCH 0/4] drm/panel: sitronix-st7789v: add support for partial mode

From: Neil Armstrong
Date: Thu Aug 03 2023 - 08:35:09 EST

On 03/08/2023 13:43, Maxime Ripard wrote:
On Thu, Aug 03, 2023 at 12:26:03PM +0200, Daniel Vetter wrote:
On Thu, 3 Aug 2023 at 11:22, Maxime Ripard <mripard@xxxxxxxxxx> wrote:

On Thu, Aug 03, 2023 at 10:51:57AM +0200, Daniel Vetter wrote:
On Thu, Aug 03, 2023 at 10:48:57AM +0200, Maxime Ripard wrote:
On Thu, Aug 03, 2023 at 10:11:22AM +0200, Neil Armstrong wrote:

On 18/07/2023 17:31, Michael Riesch wrote:
Hi all,

This series adds support for the partial display mode to the Sitronix
ST7789V panel driver. This is useful for panels that are partially
occluded by design, such as the Jasonic JT240MHQS-HWT-EK-E3. Support
for this particular panel is added as well.

Note: This series is already based on

I understand Maxime's arguments, but by looking closely at the code,
this doesn't look like an hack at all and uses capabilities of the
panel controller to expose a smaller area without depending on any
changes or hacks on the display controller side which is coherent.

Following's Daniel's summary we cannot compare it to TV overscan
because overscan is only on *some* displays, we can still get 100%
of the picture from the signal.

Still disagree on the fact that it only affects some display. But it's
not really relevant for that series.

See my 2nd point, from a quick grep aside from i915 hdmi support, no one
else sets all the required hdmi infoframes correctly. Which means on a
compliant hdmi tv, you _should_ get overscan. That's how that stuff is

Iirc you need to at least set both the VIC and the content type, maybe
even more stuff.

Unless all that stuff is set I'd say it's a kms driver bug if you get
overscan on a hdmi TV.

I have no doubt that i915 works there. The source of my disagreement is
that if all drivers but one don't do that, then userspace will have to
care. You kind of said it yourself, i915 is kind of the exception there.

The exception can be (and I'm sure it is) right, but still, it deviates
from the norm.

The right fix for these is sending the right infoframes, _not_ trying
to fiddle with overscan margins. Only the kernel can make sure the
right infoframes are sent out. If you try to paper over this in
userspace, you'll make the situation worse, not better (because
fiddling with overscan means you get scaling, and so rescaling
artifacts, and for hard contrasts along pixel lines that'll look like

So yeah this is a case of "most upstream hdmi drivers are broken".
Please don't try to fix kernel bugs in userspace.


I think I'll still like to have something clarified before we merge it:
if userspace forces a mode, does it contain the margins or not? I don't
have an opinion there, I just think it should be documented.

The mode comes with the margins, so if userspace does something really
funny then either it gets garbage (as in, part of it's crtc area isn't
visible, or maybe black bars on the screen), or the driver rejects it
(which I think is the case for panels, they only take their mode and
nothing else).

Panels can usually be quite flexible when it comes to the timings they
accept, and we could actually use that to our advantage, but even if we
assume that they have a single mode, I don't think we have anything that
enforces that, either at the framework or documentation levels?

Maybe more bugs? We've been slowly filling out all kinds of atomic kms
validation bugs in core/helper code because as a rule of thumb,
drivers get it wrong. Developers test until things work, then call it
good enough, and very few driver teams make a serious effort in trying
to really validate all invalid input. Because doing that is an
enormous amount of work.

I think for clear-cut cases like drm_panel the fix is to just put more
stricter validation into shared code (and then if we break something,
figure out how we can be sufficiently lenient again).

Panels are kind of weird, since they essentially don't exist at all in
the framework so it's difficult to make it handle them or their state.

It's typically handled by encoders directly, so each and every driver
would need to make that check, and from a quick grep, none of them are
(for the reasons you said).

Just like for HDMI, even though we can commit to changing those facts,
it won't happen overnight, so to circle back to that series, I'd like a
comment in the driver when the partial mode is enabled that if userspace
ever pushes a mode different from the expected one, we'll add the margins.

To be fair, a majority of the panel drivers would do the wrong
init of the controller with a different mode because:
- mainly the controller model is unknown
- when it's known the datasheet is missing
- when the datasheet is here, most of the registers are missing
- and most of the time the timings are buried in the init sequence

It's sad but it's the real situation.

Only a few drivers can handle a different mode, and we should perhaps
add a flag when not set rejecting a different mode for those controllers and
mark the few ones who can handle that...
And this should be a first step before adding an atomic Panel API.


That way, if and when we come back to it, we'll know what the original
intent and semantics were.