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@xxxxxxxxxx> 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 ... -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
