lkml.org 
[lkml]   [2021]   [Sep]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v4 02/24] drm/bridge: Document the probe issue with MIPI-DSI bridges
    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@cerno.tech>
    > > ---
    > > 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@samsung.com>

    I assume you'll want me to hold off that patch before someone reviews
    the rest?

    Thanks!
    Maxime
    [unhandled content-type:application/pgp-signature]
    \
     
     \ /
      Last update: 2021-09-14 16:37    [W:3.025 / U:0.060 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site