Hi, On Mon, Sep 13, 2021 at 08:29:37AM +0200, Andrzej Hajda wrote: > > 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. Yeah, there's so many moving parts it's definitely not great. > 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. Wow, that sounds like a massive change indeed :/ > Anyway: > > Reviewed-by: Andrzej Hajda <a.hajda@xxxxxxxxxxx> I assume you'll want me to hold off that patch before someone reviews the rest? Thanks! Maxime
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