linux-kernel - Re: [PATCH v2] drm/panel: rpi-touchscreen: Add backlight support

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Date:   Thu, 20 Dec 2018 20:06:45 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Gordon Hollingworth <gordon@...pberrypi.org>,
        Eric Anholt <eric@...olt.net>
Cc:     David Airlie <airlied@...ux.ie>,
        Thierry Reding <thierry.reding@...il.com>, afaerber@...e.de,
        agraf@...e.de, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        stefan.wahren@...e.com
Subject: Re: [PATCH v2] drm/panel: rpi-touchscreen: Add backlight support

On Thu, 2018-12-20 at 18:36 +0000, Gordon Hollingworth wrote:
> Assuming this is using the firmware interface then yes it's fine.  If
> it is using the i2c directly then it's possible to clash with the GPU
> driving the camera

Well we're already in such a situation without this patch, as the panel
enables the back-light though the I2C lines. This is only enabled by an
overlay, it's left to the user's discretion.

As commented previously by Alex Graf and given the constraints, I think
that it would make sense to start considering both options as good
(using FW & direct I2C access). We are defaulting to the FW based one,
but providing support/overlays for the direct access doesn't seem a bad
compromise to me.

I do understand Gordon's concerns. Maybe it would be nice for the
firmware to detect device tree nodes accessing I2C0 and disable the
camera. Or providing a FW configuration option.

Kind regards,
Nicolas

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