linux-kernel - Re: spi-mem and gpio chipselects

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Message-ID: <52aaa7364a0b40caf5f74817932de9e32d148772.camel@alliedtelesis.co.nz>
Date:   Mon, 4 Nov 2019 20:23:20 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     "broonie@...nel.org" <broonie@...nel.org>
CC:     "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: spi-mem and gpio chipselects

On Tue, 2019-11-05 at 09:06 +1300, Chris Packham wrote:
> On Mon, 2019-11-04 at 12:44 +0000, Mark Brown wrote:
> > On Mon, Nov 04, 2019 at 12:35:24AM +0000, Chris Packham wrote:
> > 
> > > I'm working on a platform that has a slightly complicated scheme for
> > > SPI chip-selects using gpios[1]. The spi controller driver in this case
> > > supports the spi-mem operations which appear to bypass the generic
> > > spi_set_cs().
> > > Would there be any harm in adding calls to spi_set_cs() to spi-mem.c?
> > > Naively spi_mem_access_start() and spi_mem_access_end() seem like
> > > convenient places to start.
> > 
> > That's only going to work in cases where the controller translates
> > things into a single SPI operation on the flash which I'm not sure is
> > always going to be the case.  We'd need a way to guarantee that the
> > controller is going to do that in order to avoid data corruption issues.
> 
> In my particular case (spi-bcm-qspi.c) bcm_qspi_bspi_exec_mem_op() does
> seem to assert the native chip-select then do it's operation. As I
> understand the wait_for_completion_timeout() will schedule so other
> tasks may run but spi_mem_access_start() has taken an io_mutex so
> anything that accesses that spi bus will block.

If we do decide that spi-mem ops and cs_gpios are incompatible we could
probably do something that disables the ops so that the spi code falls
back to using spi_transfer.