W dniu 10.09.2021 o 12:11, Maxime Ripard pisze: > 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 | 57 +++++++++++++++++++++++++++ > 2 files changed, 63 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..7cc2d2f94ae3 100644 > --- a/drivers/gpu/drm/drm_bridge.c > +++ b/drivers/gpu/drm/drm_bridge.c > @@ -96,6 +96,63 @@ > * 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 upstream driver and the bridge driver can be > + * challenging. Indeed, there's multiple cases that needs to be > + * considered: > + * > + * - The upstream 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 upstream driver should try to probe again by returning > + * EPROBE_DEFER as long as the bridge driver hasn't probed. > + * > + * - The upstream 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 upstream 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 upstream 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 upstream 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 upstream 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 > + * MIPI-DSI host driver case) is to split the operations like this: > + * > + * - The MIPI-DSI host driver must run mipi_dsi_host_register() 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 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 MIPI-DSI host 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 upstream 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); > Nice work with documenting this initialization dance. It clearly shows that bridge API lacks better mechanism - usage of mipi dsi callbacks to get notifications about bridge appearance is ugly. It remains me my resource tracking patches which I have posted long time ago [1] - they would solve the issue in much more elegant way, described here [2]. Apparently I was not stubborn enough in promoting this solution. Anyway: Reviewed-by: Andrzej Hajda <a.hajda@xxxxxxxxxxx> [1]: https://lkml.org/lkml/2014/12/10/342 [2]: https://events19.linuxfoundation.org/wp-content/uploads/2017/12/Deferred-Problem-Issues-With-Complex-Dependencies-Between-Devices-in-Linux-Kernel-Andrzej-Hajda-Samsung.pdf Regards Andrzej
