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

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Message-ID: <8a3bdd84-2789-1b42-976f-2843320750b6@baylibre.com>
Date:   Fri, 18 Nov 2022 20:02:32 +0100
From:   Carlo Caione <carlo@...one.org>
To:     Mark Brown <broonie@...nel.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 18/11/2022 16:44, Mark Brown wrote:

>> 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?

No idea why this driver is forcing 8-bit transfers when the controller
supports 16-bits (this is what this patch is fixing).

My theory is that this driver was written with the Raspberry Pi HATs in
mind and (AFAICT) the RPi has an 8-bit only SPI controller so the driver
author didn't bother with anything different.

--
Carlo Caione