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

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Date:   Wed, 9 Nov 2016 11:54:06 -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 11:39, Marek Vasut wrote:
> 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.

Oh I see. The SS_B line is the SPI SlaveSelect for the configuration port.

This is the text from the datasheet:

"SPI Slave Select. Active Low. Includes an internal weak pull-up 
resistor to VCC_SPI during configuration. During configuration, the 
logic level sampled on this pin deter-mines the configuration mode used 
by the iCE40 device. An input when sampled at the start of 
configuration. An input when in SPI Peripheral configuration mode 
(SPI_SS_B = Low). An output when in Master SPI Flash configuration mode."

So yes - it is a "real" SPI chip-select line.

Joel