linux-kernel - Re: [PATCH 3/3 v3] drm: bridge/dw-hdmi: Move edid reading to .detect() callback

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Message-ID: <20160804230125.GZ1041@n2100.armlinux.org.uk>
Date:	Fri, 5 Aug 2016 00:01:25 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Jose Abreu <Jose.Abreu@...opsys.com>
Cc:	dri-devel@...ts.freedesktop.org,
	Carlos Palminha <CARLOS.PALMINHA@...opsys.com>,
	Archit Taneja <architt@...eaurora.org>,
	David Airlie <airlied@...ux.ie>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Takashi Iwai <tiwai@...e.de>,
	Vladimir Zapolskiy <vladimir_zapolskiy@...tor.com>,
	Thierry Reding <treding@...dia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3 v3] drm: bridge/dw-hdmi: Move edid reading to
 .detect() callback

On Thu, Aug 04, 2016 at 06:13:18PM +0100, Jose Abreu wrote:
> Hi Russell,
> 
> So, I didn't use framebuffer console but used X instead and it is
> working as it should. I think we can drop this patch. I am now
> making interoperability with DVI and I am facing the following
> scenario:
>     - I start the driver
>     - An EDID is sent which tells the driver that HDMI is NOT
> supported;
>     - The driver configures itself to a DVI mode;
> 
> Until this point everything is working as it should. But:
> 
>     - Now I send an EDID which tells the driver that HDMI is
> supported;
>     - As the EDID has the same preferred mode the user will not
> reconfigure the mode and there will be no change to HDMI mode.

The EDID should still be read, but as you say, userspace doesn't take
any action because it sees that the mode parameters are still the same,
as you have identified.

> The missing change to HDMI mode will cause the test to fail. The
> workaround that I am using is to reconfigure to another video
> mode and then configure to the preferred one but I think this
> could be fixed in the driver, right?

This one is extremely awkward - to fix it, we would need to check to
see whether we reconfigure the hardware each time we read the EDID,
and I don't think that's a particularly nice thing to do.

I'd like to hear what other DRM developers think about this issue.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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