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

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Date:   Tue, 25 Apr 2017 00:56:23 -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 ***

Hi Geert

On 04/24/2017 06:10 AM, Geert Uytterhoeven wrote:
> Hi Jiada,
>
> On Mon, Apr 24, 2017 at 2:48 PM, Jiada Wang<jiada_wang@...tor.com>  wrote:
>> On 04/24/2017 03:55 AM, Geert Uytterhoeven wrote:
>>> 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.
> So "A<=>  B" and "B<=>  C" are two distinct SPI buses?
> Or do they share some signals?
>
> Your comment seems to suggest otherwise:
the use case of
"A (master) <=> B (slave)", "B (master) <=> C(slave)", do share MISO and 
MOSI lines,
but there is no SS line between A and C. so for each SPI slave device, 
there is only one
master device.

so I think the question becomes whether the above mentioned hardware 
setup is valid or not.

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 they are distinct, it should work. Then B has two SPI controllers: one slave
> controller controlled by A, and one master controller to control C.
>
> 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