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From: Maxime Ripard <maxime@cerno.tech>
To: Jonas Karlman <jonas@kwiboo.se>, Sam Ravnborg <sam@ravnborg.org>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Daniel Vetter <daniel.vetter@intel.com>,
	David Airlie <airlied@linux.ie>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Maxime Ripard <maxime@cerno.tech>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Robert Foss <robert.foss@linaro.org>,
	Andrzej Hajda <a.hajda@samsung.com>
Cc: linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: [PATCH v3 2/8] drm/bridge: Document the probe issue with MIPI-DSI bridges
Date: Mon, 23 Aug 2021 10:47:17 +0200	[thread overview]
Message-ID: <20210823084723.1493908-3-maxime@cerno.tech> (raw)
In-Reply-To: <20210823084723.1493908-1-maxime@cerno.tech>

Interactions between bridges, panels, MIPI-DSI host and the component
framework are not trivial and can lead to probing issues when
implementing a display driver. Let's document the various cases we need
too consider, and the solution to support all the cases.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
---
 Documentation/gpu/drm-kms-helpers.rst |  6 +++
 drivers/gpu/drm/drm_bridge.c          | 58 +++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/Documentation/gpu/drm-kms-helpers.rst b/Documentation/gpu/drm-kms-helpers.rst
index 10f8df7aecc0..ec2f65b31930 100644
--- a/Documentation/gpu/drm-kms-helpers.rst
+++ b/Documentation/gpu/drm-kms-helpers.rst
@@ -157,6 +157,12 @@ Display Driver Integration
 .. kernel-doc:: drivers/gpu/drm/drm_bridge.c
    :doc: display driver integration
 
+Special Care with MIPI-DSI bridges
+----------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_bridge.c
+   :doc: special care dsi
+
 Bridge Operations
 -----------------
 
diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index baff74ea4a33..794654233cf5 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -96,6 +96,64 @@
  * documentation of bridge operations for more details).
  */
 
+/**
+ * DOC: special care dsi
+ *
+ * The interaction between the bridges and other frameworks involved in
+ * the probing of the display driver and the bridge driver can be
+ * challenging. Indeed, there's multiple cases that needs to be
+ * considered:
+ *
+ * - The display driver doesn't use the component framework and isn't a
+ *   MIPI-DSI host. In this case, the bridge driver will probe at some
+ *   point and the display driver should try to probe again by returning
+ *   EPROBE_DEFER as long as the bridge driver hasn't probed.
+ *
+ * - The display driver doesn't use the component framework, but is a
+ *   MIPI-DSI host. The bridge device uses the MIPI-DCS commands to be
+ *   controlled. In this case, the bridge device is a child of the
+ *   display device and when it will probe it's assured that the display
+ *   device (and MIPI-DSI host) is present. The display driver will be
+ *   assured that the bridge driver is connected between the
+ *   &mipi_dsi_host_ops.attach and &mipi_dsi_host_ops.detach operations.
+ *   Therefore, it must run mipi_dsi_host_register() in its probe
+ *   function, and then run drm_bridge_attach() in its
+ *   &mipi_dsi_host_ops.attach hook.
+ *
+ * - The display driver uses the component framework and is a MIPI-DSI
+ *   host. The bridge device uses the MIPI-DCS commands to be
+ *   controlled. This is the same situation than above, and can run
+ *   mipi_dsi_host_register() in either its probe or bind hooks.
+ *
+ * - The display driver uses the component framework and is a MIPI-DSI
+ *   host. The bridge device uses a separate bus (such as I2C) to be
+ *   controlled. In this case, there's no correlation between the probe
+ *   of the bridge and display drivers, so care must be taken to avoid
+ *   an endless EPROBE_DEFER loop, with each driver waiting for the
+ *   other to probe.
+ *
+ * The ideal pattern to cover the last item (and all the others in the
+ * display driver case) is to split the operations like this:
+ *
+ * - In the display driver must run mipi_dsi_host_register() and
+ *   component_add in its probe hook. It will make sure that the
+ *   MIPI-DSI host sticks around, and that the driver's bind can be
+ *   called.
+ *
+ * - In its probe hook, the bridge driver must try to find its MIPI-DSI
+ *   host, register as a MIPI-DSI device and attach the MIPI-DSI device
+ *   to its host. The bridge driver is now functional.
+ *
+ * - In its &struct mipi_dsi_host_ops.attach hook, the display driver
+ *   can now add its component. Its bind hook will now be called and
+ *   since the bridge driver is attached and registered, we can now look
+ *   for and attach it.
+ *
+ * At this point, we're now certain that both the display driver and the
+ * bridge driver are functional and we can't have a deadlock-like
+ * situation when probing.
+ */
+
 static DEFINE_MUTEX(bridge_lock);
 static LIST_HEAD(bridge_list);
 
-- 
2.31.1


  parent reply	other threads:[~2021-08-23  8:47 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-23  8:47 [PATCH v3 0/8] drm/bridge: Make panel and bridge probe order consistent Maxime Ripard
2021-08-23  8:47 ` [PATCH v3 1/8] drm/bridge: Add documentation sections Maxime Ripard
2021-08-24  8:06   ` Andrzej Hajda
2021-08-23  8:47 ` Maxime Ripard [this message]
2021-08-23 16:32   ` [PATCH v3 2/8] drm/bridge: Document the probe issue with MIPI-DSI bridges Andrzej Hajda
2021-08-26  8:56     ` Maxime Ripard
2021-08-23  8:47 ` [PATCH v3 3/8] drm/mipi-dsi: Create devm device registration Maxime Ripard
2021-08-24 13:03   ` Andrzej Hajda
2021-08-23  8:47 ` [PATCH v3 4/8] drm/mipi-dsi: Create devm device attachment Maxime Ripard
2021-08-24 13:04   ` Andrzej Hajda
2021-08-23  8:47 ` [PATCH v3 5/8] drm/bridge: ps8640: Switch to devm MIPI-DSI helpers Maxime Ripard
2021-08-23  8:47 ` [PATCH v3 6/8] drm/bridge: ps8640: Register and attach our DSI device at probe Maxime Ripard
2021-08-24 13:14   ` Andrzej Hajda
2021-08-23  8:47 ` [PATCH v3 7/8] drm/bridge: sn65dsi83: Switch to devm MIPI-DSI helpers Maxime Ripard
2021-08-23  8:47 ` [PATCH v3 8/8] drm/bridge: sn65dsi83: Register and attach our DSI device at probe Maxime Ripard

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