linux-kernel - Re: [PATCH v2 0/8] drm/bridge: Make panel and bridge probe order consistent

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Message-ID: <cc638e20-aa7c-7014-f70b-1bb68e629d87@samsung.com>
Date:   Wed, 4 Aug 2021 16:09:38 +0200
From:   "a.hajda" <a.hajda@...sung.com>
To:     Maxime Ripard <maxime@...no.tech>, Sam Ravnborg <sam@...nborg.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jonas Karlman <jonas@...boo.se>,
        Robert Foss <robert.foss@...aro.org>
Cc:     linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 0/8] drm/bridge: Make panel and bridge probe order
 consistent

Hi Maxime,

I have been busy with other tasks, and I did not follow the list last 
time, so sorry for my late response.

On 28.07.2021 15:32, Maxime Ripard wrote:
> Hi,
> 
> We've encountered an issue with the RaspberryPi DSI panel that prevented the
> whole display driver from probing.
> 
> The issue is described in detail in the commit 7213246a803f ("drm/vc4: dsi:
> Only register our component once a DSI device is attached"), but the basic idea
> is that since the panel is probed through i2c, there's no synchronization
> between its probe and the registration of the MIPI-DSI host it's attached to.
> 
> We initially moved the component framework registration to the MIPI-DSI Host
> attach hook to make sure we register our component only when we have a DSI
> device attached to our MIPI-DSI host, and then use lookup our DSI device in our
> bind hook.
> 
> However, all the DSI bridges controlled through i2c are only registering their
> associated DSI device in their bridge attach hook, meaning with our change

I guess this is incorrect. I have promoted several times the pattern 
that device driver shouldn't expose its interfaces (for example 
component_add, drm_panel_add, drm_bridge_add) until it gathers all 
required dependencies. In this particular case bridges should defer 
probe until DSI bus becomes available. I guess this way the patch you 
reverts would work.

I advised few times this pattern in case of DSI hosts, apparently I 
didn't notice the similar issue can appear in case of bridges. Or there 
is something I have missed???

Anyway there are already eleven(?) bridge drivers using this pattern. I 
wonder if fixing it would be difficult, or if it expose other issues???
The patches should be quite straightforward - move 
of_find_mipi_dsi_host_by_node and mipi_dsi_device_register_full to probe 
time.

Finally I think that if we will not fix these bridge drivers we will 
encounter another set of issues with new platforms connecting "DSI host 
drivers assuming this pattern" and "i2c/dsi device drivers assuming 
pattern already present in the bridges".

Regards
Andrzej