linux-kernel - Re: [PATCH 2/3] drm/tiny: ili9486: Do not assume 8-bit only SPI controllers

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Message-ID: <Y3eoYTZRyRJnze1z@sirena.org.uk>
Date:   Fri, 18 Nov 2022 15:44:33 +0000
From:   Mark Brown <broonie@...nel.org>
To:     Carlo Caione <carlo@...one.org>
Cc:     Kamlesh Gurudasani <kamlesh.gurudasani@...il.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Jerome Brunet <jbrunet@...libre.com>,
        David Airlie <airlied@...il.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Kevin Hilman <khilman@...libre.com>,
        Daniel Vetter <daniel@...ll.ch>,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/3] drm/tiny: ili9486: Do not assume 8-bit only SPI
 controllers

On Fri, Nov 18, 2022 at 11:36:27AM +0100, Carlo Caione wrote:
> On 17/11/2022 15:59, Mark Brown wrote:

> > So this is an issue in the MIPI DBI code where the interpretation of the
> > buffer passed in depends on both the a caller parameter and the
> > capabilities of the underlying SPI controller, meaning that a driver can
> > suddenly become buggy when used with a new controller?

> The MIPI DBI code is fine, in fact it is doing the correct thing in the
> mipi_dbi_typec3_command() function. The problem is that the ILI9486
> driver is hijacking that function installing its own hook that is wrong.

Ah, I see - it's causing confusion because it peers into the
internals of the underlying code.

> The problem arrives when your controller does support 16-bits, so your
> data is not swapped, but you still put the data on the bus with 8-bit
> transfers.

Why would you need to use 8 bit transfers if the controller
supports 16 bits?

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