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

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Date:   Wed, 9 Nov 2016 19:39:59 +0100
From:   Marek Vasut <marex@...x.de>
To:     Joel Holdsworth <joel@...webreathe.org.uk>,
        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 11/09/2016 07:37 PM, Joel Holdsworth wrote:
> 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:

No. I am asking whether the signal which is INPUT on the iCE40 side is
really a chipselect signal for the SPI bus OR something which mostly
behaves/looks like a chipselect but is not really a chipselect.

> 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.

This is not relevant. FYI: using separate GPIO as a SPI chip select has
it's own problems.

-- 
Best regards,
Marek Vasut