Re: [PATCH] spi: Make GPIO CSs honour the SPI_NO_CS flag

[Trent Piepho]

On Tue, 2018-10-16 at 10:03 +0100, Mark Brown wrote:
> On Mon, Oct 15, 2018 at 06:34:18PM +0000, Trent Piepho wrote:
> 
> > What about the calls to spi->controller->set_cs() after this? Should a
> > driver provided set_cs method be responsible for checking SPI_NO_CS? 
> > Or should it not be called in the first place?
> 
> This seems like something that should be done entirely in the framework,
> no point in every single driver having to open code the same thing.
> 
> > I imagine it depends on what set_cs needs to do, which might not be
> > solely related to changing the CS line.
> 
> It should be.  If something is hanging other work on set_cs() then it's
> going to break.

IIRC, for spi-dw setting CS is the only way to trigger the master to do
anything.  I think orion is the same way.  Even if you don't want a CS
line the driver still needs to assert one.  Which CS to use as the
dummy CS is a challenge that has come up before.

bcm2835_spi_set_cs() does check SPI_NO_CS, but it still does a lot of
other stuff even if that is set, likely because of the above issue.