linux-kernel - Re: [PATCH] drm/vc4: dsi: Only register our component once a DSI device is attached

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Message-ID: <20200709073358.zeeczn6253na36l6@gilmour.lan>
Date:   Thu, 9 Jul 2020 09:33:58 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Eric Anholt <eric@...olt.net>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        linux-rpi-kernel@...ts.infradead.org,
        bcm-kernel-feedback-list@...adcom.com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        Tim Gover <tim.gover@...pberrypi.com>,
        Phil Elwell <phil@...pberrypi.com>,
        Andrzej Hajda <a.hajda@...sung.com>
Subject: Re: [PATCH] drm/vc4: dsi: Only register our component once a DSI
 device is attached

Hi Eric,

On Tue, Jul 07, 2020 at 09:48:45AM -0700, Eric Anholt wrote:
> On Tue, Jul 7, 2020 at 3:26 AM Maxime Ripard <maxime@...no.tech> wrote:
> >
> > If the DSI driver is the last to probe, component_add will try to run all
> > the bind callbacks straight away and return the error code.
> >
> > However, since we depend on a power domain, we're pretty much guaranteed to
> > be in that case on the BCM2711, and are just lucky on the previous SoCs
> > since the v3d also depends on that power domain and is further in the probe
> > order.
> >
> > In that case, the DSI host will not stick around in the system: the DSI
> > bind callback will be executed, will not find any DSI device attached and
> > will return EPROBE_DEFER, and we will then remove the DSI host and ask to
> > be probed later on.
> >
> > But since that host doesn't stick around, DSI devices like the RaspberryPi
> > touchscreen whose probe is not linked to the DSI host (unlike the usual DSI
> > devices that will be probed through the call to mipi_dsi_host_register)
> > cannot attach to the DSI host, and we thus end up in a situation where the
> > DSI host cannot probe because the panel hasn't probed yet, and the panel
> > cannot probe because the DSI host hasn't yet.
> >
> > In order to break this cycle, let's wait until there's a DSI device that
> > attaches to the DSI host to register the component and allow to progress
> > further.
> >
> > Suggested-by: Andrzej Hajda <a.hajda@...sung.com>
> > Signed-off-by: Maxime Ripard <maxime@...no.tech>
> 
> I feel like I've written this patch before, but I've thankfully
> forgotten most of my battle with DSI probing.  As long as this still
> lets vc4 probe in the absence of a DSI panel in the DT as well, then
> this is enthusiastically acked.

I'm not really sure what you mean by that, did you mean vc4 has to probe
when the DSI controller is enabled but there's no panel described, or it
has to probe when the DSI controller is disabled?

Maxime