Re: [PATCH v8 3/3] fpga: Add support for Lattice iCE40 FPGAs

[Joel Holdsworth]

On 09/11/16 05:01, Marek Vasut wrote:
> On 11/08/2016 06:30 PM, Joel Holdsworth wrote:
> > > > > On the whole, I don't think the zero-length transfers are too
> > > > > egregiously bad, and all the alternatives seem worse to me.
> > > >
> > > > So why not turn the CS line into GPIO and just toggle the GPIO?
> > >
> > > Does that work with *all* SPI controllers?
> >
> > It does not - no. See my other email.
>
> And is that line an actual CS of that lattice chip or a generic input
> which almost works like CS?

I mean a generic output vs. a special CS output built into the SPI 
master of the application processor. Take a look at how spi_set_cs(..) 
works:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/spi/spi.c?id=refs/tags/v4.9-rc4#n695

static void spi_set_cs(struct spi_device *spi, bool enable)
{
	if (spi->mode & SPI_CS_HIGH)
		enable = !enable;

	if (gpio_is_valid(spi->cs_gpio))
		gpio_set_value(spi->cs_gpio, !enable);
	else if (spi->master->set_cs)
		spi->master->set_cs(spi, !enable);
}

So on some SPI masters, spi->master->set_cs is handled separately from 
normal GPIOs. Hence why I want to use this machinery, rather than doing 
it with a GPIO.

Joel