Hi Maxime,
On 23.08.2021 10:47, 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
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.
I guess component_add is leftover from previous iteration (as you wrote few lines below) component_add should be called from dsi host attach callback.
- 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.
- */
Beside small mistake the whole patch looks OK for me. Maybe it would be worth to mention what is the real cause of this "special DSI case" - there is mutual dependency between two following entities in display chain:
1. display driver - it provides DSI bus, and requires drm_bridge or drm_panel provided by child device.
2. bridge or panel with DSI transport - it requires DSI bus provided by display driver, and provides drm_bridge or drm_panel interface required by display driver.
I guess similar issues can appear with other data/control bus-es, apparently DSI case is the most common.
And one more thing - you use "display driver" term but this is also case of any bridge providing DSI bus - there are already 3 such bridges in kernel - cdns, nwl, synopsys, tc358768, maybe "DSI host" would be better term.
And another thing - downstream device can be bridge or *panel*, it would be good to mention that panels also should follow this pattern.
Btw this is another place where word bridge can be 1:1 replaced by word panel - it clearly suggest that DRM subsystem waits for brave men who proposes patches unifying them, we would save lot of words, and lines of code if we could use drm_sink instead of "if (sink is bridge) do sth else do sth-similar-but-with-drm_panel-interface".
Regards
Andrzej
static DEFINE_MUTEX(bridge_lock); static LIST_HEAD(bridge_list);