linux-kernel - Re: [PATCH 0/2] drm/bridge: dw-hdmi: disable loading of DW-HDMI CEC sub-driver

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Message-ID: <YH9agHwLhEL27TA1@pendragon.ideasonboard.com>
Date:   Wed, 21 Apr 2021 01:49:36 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     Hans Verkuil <hverkuil@...all.nl>, jernej.skrabec@...l.net,
        jonas@...boo.se, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, robert.foss@...aro.org,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/2] drm/bridge: dw-hdmi: disable loading of DW-HDMI CEC
 sub-driver

On Tue, Apr 20, 2021 at 05:19:52PM +0200, Neil Armstrong wrote:
> On 20/04/2021 17:13, Hans Verkuil wrote:
> > On 16/04/2021 13:38, Neil Armstrong wrote:
> >> On 16/04/2021 11:58, Laurent Pinchart wrote:
> >>> Hi Neil,
> >>>
> >>> On Fri, Apr 16, 2021 at 11:27:35AM +0200, Neil Armstrong wrote:
> >>>> This adds DW-HDMI driver a glue option to disable loading of the CEC sub-driver.
> >>>>
> >>>> On some SoCs, the CEC functionality is enabled in the IP config bits, but the
> >>>> CEC bus is non-functional like on Amlogic SoCs, where the CEC config bit is set
> >>>> but the DW-HDMI CEC signal is not connected to a physical pin, leading to some
> >>>> confusion when the DW-HDMI CEC controller can't communicate on the bus.
> >>>
> >>> If we can't trust the CEC config bit, would it be better to not use it
> >>> at all, and instead let each platform glue logic tell whether to enable
> >>> CEC or not ?
> >>
> >> Actually, the CEC config bit is right, the HW exists and should be functional, but
> >> this bit doesn't tell if the CEC signal is connected to something.
> >>
> >> This lies in the IP integration, like other bits under the "amlogic,meson-*-dw-hdmi"
> >> umbrella.
> >>
> >> The first attempt was by Hans using DT, but adding a property in DT for a vendor
> >> specific compatible doesn't make sense. Another idea would be to describe the
> >> CEC signal endpoint like we do for video signal, but I think this is out of scope and
> >> this solution is much simpler and straightforward, and it's more an exception than
> >> a general use case to solve.
> > 
> > While a DT property might not make sense in this particular case, I still
> > believe that it is a perfectly valid approach in general: whether or not
> > the CEC pin is connected is at the hardware level decision, it is not
> > something that software can detect. If the designer of the board didn't
> > connect it, then the only place you can define that is in the device tree.
> 
> Agreed, we need to define a smart way to declare CEC bus relationship in DT, the side
> effect would be to handle this particular case.

I wonder if it would make sense to use the OF graph bindings to describe
the connection between the CEC controller and the CEC "device" (which I
assume in most cases will be a DT node for a physical connector). Or is
this overkill ?

> > Anyway, for meson I am fine with this solution. At least it prevents creating
> > a non-functioning cec device. So for this series:
> > 
> > Acked-by: Hans Verkuil <hverkuil-cisco@...all.nl>
> 
> Thanks,
> 
> Applying this serie to drm-misc-next
> 
> >>>> Jernej Skrabec (1):
> >>>>   drm/bridge/synopsys: dw-hdmi: Add an option to suppress loading CEC
> >>>>     driver
> >>>>
> >>>> Neil Armstrong (1):
> >>>>   drm/meson: dw-hdmi: disable DW-HDMI CEC sub-driver
> >>>>
> >>>>  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 2 +-
> >>>>  drivers/gpu/drm/meson/meson_dw_hdmi.c     | 1 +
> >>>>  include/drm/bridge/dw_hdmi.h              | 2 ++
> >>>>  3 files changed, 4 insertions(+), 1 deletion(-)

-- 
Regards,

Laurent Pinchart