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

From: Neil Armstrong
Date: Thu Aug 03 2023 - 05:31:08 EST

On 03/08/2023 11:22, Maxime Ripard 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.

HDMI spec is hidden behind a paywall, HDMI testing is a mess, HDMI real
implementation on TVs and Displays is mostly broken, and HDMI certification
devices are too expensive... this is mainly why only i915 handles it correctly.

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?

Yep, this is why we would need a better atomic based panel API that would
permit us handling dynamic timings for panel and get out of the single-mode
for modern panels.