On Wed, Jul 21, 2021 at 02:05:01PM +0200, Daniel Vetter wrote:
On Tue, Jul 20, 2021 at 03:45:19PM +0200, Maxime Ripard wrote:
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
I still have this dream that eventually we resurrect a patch to add device_link to bridges/panels (ideally automatically), to help with some of the suspend/resume issues around here.
Will this make things worse?
I think it'd be really good to figure that out with some coding, since if we have incompatible solution to handle probe issues vs suspend/resume issues, we're screwed.
Atm the duct-tape is to carefully move things around between suspend and suspend_early hooks (and resume and resume_late) and hope it all works ...
Just remembered: The other reason for device links was module ordering fun. Especially when you have parts managed as a component. Currently if you unload a bridge driver then in some cases the drm_device doesn't rebind. -Daniel
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 c9a950bfdfe5..81f8dac12367 100644 --- a/drivers/gpu/drm/drm_bridge.c +++ b/drivers/gpu/drm/drm_bridge.c @@ -95,6 +95,66 @@
- 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
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch