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@xxxxxxxxxx> > --- > 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); >
