linux-kernel - Re: [PATCH] drm/sun4i: Only count TCON endpoints as valid outputs

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Message-ID: <20161122153730.nj5fl34oleu2uylz@lukather>
Date:   Tue, 22 Nov 2016 16:37:30 +0100
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     David Airlie <airlied@...ux.ie>,
        linux-sunxi <linux-sunxi@...glegroups.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/sun4i: Only count TCON endpoints as valid outputs

Hi,

On Fri, Nov 18, 2016 at 10:22:40AM +0800, Chen-Yu Tsai wrote:
> On Fri, Nov 18, 2016 at 3:02 AM, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> > On Wed, Nov 16, 2016 at 05:37:31PM +0800, Chen-Yu Tsai wrote:
> >> The sun4i DRM driver counts the number of endpoints it found and
> >> registers the whole DRM pipeline if any endpoints are found.
> >>
> >> However, if the TCON and its child endpoints (LCD panels, TV encoder,
> >> HDMI encoder, MIPI DSI encoder, etc.) aren't found, that means we
> >> don't have any usable CRTCs, and the display pipeline is incomplete
> >> and useless.
> >
> > If some node set as available is not probed, then yes, it does, but
> > I'm not really sure how it's a problem. Quite the opposite actually.
> 
> Actually the problem occurs when the TCON is _not_ available, but
> the other endpoints preceding it are.

By preceding, you mean the display engine or the HDMI or TV encoders?

> >> The debug message "Queued %d outputs on pipeline %d\n" is also telling.
> >>
> >> This patch makes the driver only count enabled TCON endpoints. If
> >> none are found, the DRM pipeline is not used. This avoids screwing
> >> up the simple framebuffer provided by the bootloader in cases where
> >> we aren't able to support the display with the DRM subsystem, due
> >> to lack of panel or bridge drivers, or just lack of progress.
> >
> > The framebuffer is removed only at bind time, which means that all the
> > drivers have probed already. Lack of progress isn't an issue here,
> > since the node simply won't be there, and we wouldn't have it in the
> > component lists. And lack of drivers shouldn't be an issue either,
> > since in order for bind to be called, all the drivers would have
> > gone through their probe.
> >
> > So I'm not really sure what it fixes.
> 
> To recap, on sun6i I had enabled the display engine node by default
> in the dtsi, along with the backend and drc. The tcon is disabled
> by default, so it doesn't get added to the list of components.
> The available components get probed, binded, and simplefb gets
> pushed out.
> 
> I suppose disabling the display engine by default would be better?
> At least simplefb still works.

Yep, that works for me.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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