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@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 ... 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 -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
