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

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Date:   Wed, 9 Nov 2016 11:37:08 -0700
From:   Joel Holdsworth <joel@...webreathe.org.uk>
To:     Marek Vasut <marex@...x.de>,
        Moritz Fischer <moritz.fischer@...us.com>
Cc:     Alan Tull <atull@...nsource.altera.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Rob Herring <robh@...nel.org>,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-spi@...r.kernel.org, Clifford Wolf <clifford@...fford.at>
Subject: Re: [PATCH v8 3/3] fpga: Add support for Lattice iCE40 FPGAs

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