linux-kernel - Re: [PATCH] spi: Avoid setting the chip select if we don't need to

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Message-ID: <20200701093316.GA5385@sirena.org.uk>
Date:   Wed, 1 Jul 2020 10:33:16 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
Cc:     "dianders@...omium.org" <dianders@...omium.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "swboyd@...omium.org" <swboyd@...omium.org>,
        "linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
        "enric.balletbo@...labora.com" <enric.balletbo@...labora.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bleung@...omium.org" <bleung@...omium.org>
Subject: Re: [PATCH] spi: Avoid setting the chip select if we don't need to

On Wed, Jul 01, 2020 at 06:26:24AM +0000, Ardelean, Alexandru wrote:
> On Mon, 2020-06-29 at 16:41 -0700, Douglas Anderson wrote:

> > +	spi->controller->last_cs_enable = enable;
> > +	spi->controller->last_cs_mode_high = spi->mode & SPI_CS_HIGH;

> I don't feel like this is the best approach for the SPI CS handling,
> because it's pretty difficult to guess the last CS state, and whether this
> return would cause other weirder issues [like not toggling CS when it
> should].

There's no guesswork involved here - the only thing that should be
setting the chip select is the SPI core so other than at startup we
always know the state of the chip select.

> Maybe a question is: when should this CS be toggled [or not]?
> Is it between 2 calls of spi_transfer_one_message() or between 2
> spi_transfers?
> Or, is "xfer->cs_change == 1" where it shouldn't be?

This is well documented, it's asserted while a message is being
transferred unless changed by cs_change in which case we do whatever the
opposite of the default action is.

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