linux-kernel - Re: [PATCH RFC 0/5] *** SPI Slave mode support ***

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Date:   Mon, 24 Apr 2017 05:48:48 -0700
From:   Jiada Wang <jiada_wang@...tor.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Mark Brown <broonie@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <kernel@...gutronix.de>,
        Fabio Estevam <fabio.estevam@....com>,
        linux-spi <linux-spi@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC 0/5] *** SPI Slave mode support ***

Hello Geert

On 04/24/2017 03:55 AM, Geert Uytterhoeven wrote:
> Hi Jiada,
>
> On Fri, Apr 14, 2017 at 7:39 AM, Jiada Wang<jiada_wang@...tor.com>  wrote:
>> On 04/13/2017 12:47 PM, Geert Uytterhoeven wrote:
>>> On Thu, Apr 13, 2017 at 2:59 PM, Mark Brown<broonie@...nel.org>   wrote:
>>>> On Thu, Apr 13, 2017 at 05:13:59AM -0700, jiada_wang@...tor.com wrote:
>>>>> From: Jiada Wang<jiada_wang@...tor.com>
>>>>>
>>>>> v1:
>>>>>     add Slave mode support in SPI core
>>>>>     spidev create slave device when SPI controller work in slave mode
>>>>>     spi-imx support to work in slave mode
>>>> Adding Geert who also had a series doing this in progress that was
>>>> getting very near to being merged.
>>> Thank you!
>>>
>>> Actually my plan is to fix the last remaining issues and resubmit for
>>> v4.13.
>> I noticed your patch set for SPI slave support,
>> (I am sure you can find out some of the change
>> in this patch set is based on your work).
>> we have similar requirement to add slave mode support to ecspi IP on imx6
>> Soc.
>>
>> Our use case is to use spidev as an interface to communicate with external
>> SPI master devices.
>> meanwhile the SPI bus controller can also act as master device to send data
>> to other
>> SPI slave devices on the board.
> That sounds a bit hackish to me. SPI was never meant to be a multi-master bus.
> While it can be done, you will need external synchronization (signals) to
> avoid conflicts between the SPI masters.
It doesn't need to be a multi-master bus,
for example A is master device for slave device B.
while B has its own slave device C
for each SPI connection A <=> B, and B <=> C, there is only one master 
device.

and I think from use case point of view, it's very normal,
one CPU upon receives command from external SPI master device,
it writes data to its own slave device (EEPROM) connected to it.

Thanks,
Jiada
>> I found in your implementation, SPI bus controller is limited to either work
>> in master mode or
>> slave mode, is there any reasoning to not configure SPI mode based on SPI
>> devices use case?
> If you really need both master and slave support, you can use 2 subnodes
> in DT, the first representing the master, the second the slave.
>
> Mark, what's your opinion about this?
>
> Thanks!
>
> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds