linux-kernel - Re: [PATCH 2/4] spi: add new SPI_CS

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Message-ID: <20180718150450.GO5700@sirena.org.uk>
Date:   Wed, 18 Jul 2018 16:04:50 +0100
From:   Mark Brown <broonie@...nel.org>
To:     David Lechner <david@...hnology.com>
Cc:     linux-spi@...r.kernel.org, linux-iio@...r.kernel.org,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] spi: add new SPI_CS_WORD flag

On Mon, Jul 16, 2018 at 10:20:50PM -0500, David Lechner wrote:
> This adds a new SPI mode flag, SPI_CS_WORD, that is used to indicate
> that a SPI device requires the chip select to be toggled after each
> word that is transferred.

This feels like it should have a soft implementation if it is going to
be truly usable, the vast majority of SPI controllers don't do this and
I can only think of a few that have the hardware feature.  I'd also
expect to see some validation added to the core spi_setup() since at
present a client driver could set the mode option but then have it
ignored by the controller which would presumably break things, we
currently only have checks for specific modes and nothing that'd catch
an unknown flag like this.

Ideally we'd also have some ability to use this as an optimization where
possible with longer sequences (I can see a regmap cache sync being able
to take advantage of this for example) but that might be more trouble
than it's worth.

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