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

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Date:   Tue, 7 Jul 2020 09:48:45 -0700
From:   Eric Anholt <eric@...olt.net>
To:     Maxime Ripard <maxime@...no.tech>
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

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.