From: Maxime Ripard <email@example.com> To: Sam Ravnborg <firstname.lastname@example.org>, Daniel Vetter <email@example.com>, David Airlie <firstname.lastname@example.org>, Thierry Reding <email@example.com>, Laurent Pinchart <Laurent.firstname.lastname@example.org>, Maarten Lankhorst <email@example.com>, Thomas Zimmermann <firstname.lastname@example.org>, Maxime Ripard <email@example.com>, Jernej Skrabec <firstname.lastname@example.org>, Neil Armstrong <email@example.com>, Andrzej Hajda <firstname.lastname@example.org>, Jonas Karlman <email@example.com>, Robert Foss <firstname.lastname@example.org> Cc: email@example.com, firstname.lastname@example.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: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> 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 <email@example.com> --- 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
next prev 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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