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

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Date:   Thu, 10 Nov 2016 13:11:08 +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 07:54 PM, Joel Holdsworth wrote:
> 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.

OK, thanks for checking.

-- 
Best regards,
Marek Vasut