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

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Message-ID: <0821533b-9e6a-237e-3845-741e82de4ed9@baylibre.com>
Date:   Tue, 20 Apr 2021 17:19:52 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     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 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.

> 
> 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

Neil

> 
> Regards,
> 
> 	Hans
> 
>>
>> Neil
>>
>>>
>>>> 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(-)
>>>>
>>>
>>
>> _______________________________________________
>> dri-devel mailing list
>> dri-devel@...ts.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/dri-devel
>>
> 

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