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Date:   Thu, 3 Aug 2023 10:51:57 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Maxime Ripard <mripard@kernel.org>
Cc:     Neil Armstrong <neil.armstrong@linaro.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        Michael Riesch <michael.riesch@wolfvision.net>,
        Sam Ravnborg <sam@ravnborg.org>,
        Sebastian Reichel <sre@kernel.org>,
        Gerald Loacker <gerald.loacker@wolfvision.net>,
        David Airlie <airlied@gmail.com>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Conor Dooley <conor+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH 0/4] drm/panel: sitronix-st7789v: add support for partial
 mode
Message-ID: <ZMtqraOyGN9JvVj9@phenom.ffwll.local>
Mail-Followup-To: Maxime Ripard <mripard@kernel.org>,
        Neil Armstrong <neil.armstrong@linaro.org>,
        Michael Riesch <michael.riesch@wolfvision.net>,
        Sam Ravnborg <sam@ravnborg.org>, Sebastian Reichel <sre@kernel.org>,
        Gerald Loacker <gerald.loacker@wolfvision.net>,
        David Airlie <airlied@gmail.com>,
        Miquel Raynal <miquel.raynal@bootlin.com>,
        Conor Dooley <conor+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Rob Herring <robh+dt@kernel.org>, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
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 <292c3e7d-82ea-2631-bd4b-ef747f56287c@linaro.org>
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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:
> > Hi,
> > 
> > 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
> > > https://lore.kernel.org/lkml/20230714013756.1546769-1-sre@kernel.org/
> > 
> > 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
speced.

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 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).

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