From: Marek Vasut <marek.vasut@gmail.com>
To: "Bastelli Carlo" <carlo_bastelli@yahoo.it>
Subject: Re: SLAVE Side SPI kernel driver development
Date: Sat, 25 Aug 2012 13:44:10 +0200
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Cc: "Ned Forrester" <nforrester@whoi.edu>,
        "Fabio Estevam" <festevam@gmail.com>,
        spi-devel-general@lists.sourceforge.net, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
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Dear Bastelli Carlo,

> In my application in fact the slave is connected to the master without
> other devices on the bus, a simple point to point, and receives a frame of
> known dimensions.

What hardware do you use? What CPU is it?

> I think I will be very busy to read post in the coming days  :)

Good luck with that.

> Thanks to all of you who provided suggestions.
> Carlo Bastelli
> 
> ----- Original Message -----
> From: "Ned Forrester" <nforrester@whoi.edu>
> To: "Marek Vasut" <marex@denx.de>
> Cc: "Fabio Estevam" <festevam@gmail.com>;
> <spi-devel-general@lists.sourceforge.net>; "Bastelli Carlo (yahoo)"
> <carlo_bastelli@yahoo.it>; <linux-kernel@vger.kernel.org>;
> <linux-arm-kernel@lists.infradead.org>
> Sent: Friday, August 24, 2012 6:54 PM
> Subject: Re: SLAVE Side SPI kernel driver development
> 
> On 08/24/2012 11:38 AM, Marek Vasut wrote:
> > Dear Fabio Estevam,
> > 
> >> On Fri, Aug 24, 2012 at 11:57 AM, Bastelli Carlo (yahoo)
> > 
> > CCing SPI list
> > 
> >> <carlo_bastelli@yahoo.it> wrote:
> >>> Hello, I'm Carlo, I have a difficult task at work, my boss asked me to
> >>> develop
> >>> 
> >>>  a driver SPI slave side on embedded ARM processor running Linux.
> >>> 
> >>> Precisely linux occur on the SPI bus as a slave, not master as required
> >>> by the hierarchical structure of the current kernel. The new driver
> >>> will
> >>> 
> >>>  receive via interrupt and respond when selected by its chip select.
> > 
> > So a soft-spi ? It's gonna be slow as hell and unstable too.
> > 
> >>>  Can
> >>>  anyone suggest the best way to develop this driver?
> > 
> > Get a CPU which has SPI-slave capable IP, start from there. If you were
> > to implement soft-spi this way, you'd have to use FIQ on ARM, to be able
> > to handle
> > stuff with reasonable latency ... check how the ps/2-gpio driver is done
> > in
> > Linux ... I tried it, it barely worked, you don't want to go down that
> > road.
> > 
> >>>  Personally, I
> >>> 
> >>> have identified two main roads, the first if possible to exploit the
> >>> structure SPI master driver
> > 
> > Not happening. You'd have to split the parts from the struct spi_master
> > into
> > common and master-only parts. That's for starters.
> > 
> >>> with a few tricks that now I can not
> >>> define. The second most radical, write a driver for a UART like, but
> >>> managing the SPI device.
> >>> Thank you for any suggestions or examples.
> > 
> > First of all, the SPI slave mode is COMPLETELY different from the master
> > mode.
> > You need to react on incoming CS-low ASAP, which might turn out to be a
> > problem
> > on it's own. Then, even if you do, you do not know how long the transfer
> > is
> > going to be, so you need to allocate enough buffers as the transfer
> > progresses
> > onwards.
> > 
> > In case of your soft-spi, how do you plan to sample the CLK line to sync
> > clock?
> > 
> >> Marek got spi slave running on a mx28. See his patch:
> >> http://www.spinics.net/lists/arm-kernel/msg190860.html
> > 
> > This was tested (and developed) on this board:
> > http://www.denx-cs.de/?q=M28
> > 
> > The userland code is really easy too, simply pump it into spidev or read
> > from
> > it. But all that is only possible because of the SPI IP in the mx28 is
> > nice.
> > 
> >> Regards,
> >> 
> >> Fabio Estevam
> > 
> > Best regards,
> > Marek Vasut
> 
> I generally agree with the above comments.  A generalized SPI slave must
> be able to respond instantly, often within one clock cycle.  This can
> only work in hardware, or in an OS like Linux when the activity on the
> bus is completely predictable by the Linux slave.  The only cases I know
> of are streaming of data in a predictable way between a single master
> and single slave (no other devices on the bus).
> 
> This topic comes up periodically, but I don't think there has been any
> change in the situation for all the time I've followed the discussion.
> I recommend that you read the archives of this mailing list to
> understand the limitations of a potential slave mode.  The archives can
> be found at:
> 
> http://sourceforge.net/mailarchive/forum.php?forum_name=spi-devel-general
> 
> then look for threads with "slave" in the subject in the following
> months (I'd give thread start dates, but parts of the threads are
> disconnected in the archives, and this is the easiest way to see all the
> parts, day ranges are in parentheses):
> 
> 2007/04 (13-18)
> 2007/09/27 and 2007/10 (13-17)
> 2009/03 (2-3)
> 2009/12/18 and 2010/1/19 and 2010/02 (14-16)
> 2010/07 (26) and 2010/08 (13)
> 2011/01 (28-31) and 2011/02 (1-10)
> 2011/10 (18)
> 2012/06 (29-30 and 2012/07 (1-17)

Best regards,
Marek Vasut
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