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Date:   Thu, 3 Aug 2023 16:55:40 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Maxime Ripard <mripard@kernel.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        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
Subject: Re: [PATCH 0/4] drm/panel: sitronix-st7789v: add support for partial
 mode
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Mail-Followup-To: Maxime Ripard <mripard@kernel.org>,
        Neil Armstrong <neil.armstrong@linaro.org>,
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        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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On Thu, Aug 03, 2023 at 01:43:08PM +0200, 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@kernel.org> 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:
> > > > > > 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 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
> > crap).
> > 
> > So yeah this is a case of "most upstream hdmi drivers are broken".
> > Please don't try to fix kernel bugs in userspace.
> 
> ACK.
> 
> > > > > 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).

I think the panel bridge driver infra is the right spot to put this, and
then push drivers a bit more towards using that.

Because yeah if they hand-roll the panel integration, they'll probably
miss a lot of these details :-/
-Sima

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


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