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From: Maxime Ripard <maxime@cerno.tech>
To: Sam Ravnborg <sam@ravnborg.org>,
	Daniel Vetter <daniel.vetter@intel.com>,
	David Airlie <airlied@linux.ie>,
	Thierry Reding <thierry.reding@gmail.com>,
	Laurent Pinchart <Laurent.pinchart@ideasonboard.com>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Maxime Ripard <maxime@cerno.tech>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Andrzej Hajda <a.hajda@samsung.com>,
	Jonas Karlman <jonas@kwiboo.se>,
	Robert Foss <robert.foss@linaro.org>
Cc: linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org
Subject: [PATCH v2 4/8] drm/bridge: Document the probe issue with MIPI-DSI bridges
Date: Wed, 28 Jul 2021 15:32:25 +0200	[thread overview]
Message-ID: <20210728133229.2247965-5-maxime@cerno.tech> (raw)
In-Reply-To: <20210728133229.2247965-1-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          | 60 +++++++++++++++++++++++++++
 2 files changed, 66 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 71d3370ce209..fd01f1ce7976 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c
@@ -94,6 +94,66 @@
  * 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 not try to find its
+ *   MIPI-DSI host or register as a MIPI-DSI device. As far as the
+ *   framework is concerned, it must only call drm_bridge_add().
+ *
+ * - In its bind hook, the display driver must try to find the bridge
+ *   and return -EPROBE_DEFER if it doesn't find it. If it's there, it
+ *   must call drm_bridge_attach(). The MIPI-DSI host is now functional.
+ *
+ * - In its &drm_bridge_funcs.attach hook, the bridge driver can now try
+ *   to find its MIPI-DSI host and can register as a MIPI-DSI device.
+ *
+ * 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:[~2021-07-28 13:32 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-28 13:32 [PATCH v2 0/8] drm/bridge: Make panel and bridge probe order consistent Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 1/8] Revert "drm/vc4: dsi: Only register our component once a DSI device is attached" Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 2/8] drm/bridge: Add a function to abstract away panels Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 3/8] drm/bridge: Add documentation sections Maxime Ripard
2021-07-28 13:32 ` Maxime Ripard [this message]
2021-07-28 13:32 ` [PATCH v2 5/8] drm/panel: Create attach and detach callbacks Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 6/8] drm/vc4: dsi: Switch to drm_of_get_bridge Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 7/8] drm/panel: raspberrypi-touchscreen: Use the attach hook Maxime Ripard
2021-07-28 13:32 ` [PATCH v2 8/8] drm/panel: raspberrypi-touchscreen: Remove MIPI-DSI driver Maxime Ripard
     [not found] ` <CGME20210804140941eucas1p2d4d4ec491074530c714797523aec05ea@eucas1p2.samsung.com>
2021-08-04 14:09   ` [PATCH v2 0/8] drm/bridge: Make panel and bridge probe order consistent a.hajda
2021-08-09  8:00     ` Jagan Teki
2021-08-09 12:03       ` Andrzej Hajda
2021-08-20 16:49     ` Maxime Ripard

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