SLAVE Side SPI kernel driver development

I will use an ARM processor from Atmel, do not yet know the exact model, but 
I know for a fact that the SPI controller included in their ARM processor 
supports slave mode. When I have more details on the hardware used will not 
forget to update the forum.

Thanks & Regards

Carlo Bastelli

----- Original Message ----- 
From: "Marek Vasut" <redacted>
To: "Bastelli Carlo" <redacted>
Cc: "Ned Forrester" <redacted>; "Fabio Estevam" 
[off-list ref]; [off-list ref]; 
[off-list ref]; [off-list ref]
Sent: Saturday, August 25, 2012 1:44 PM
Subject: Re: SLAVE Side SPI kernel driver development


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" <redacted>
To: "Marek Vasut" <marex@denx.de>
Cc: "Fabio Estevam" <festevam@gmail.com>;
[off-list ref]; "Bastelli Carlo (yahoo)"
[off-list ref]; [off-list ref];
[off-list ref]
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:

quoted

Dear Fabio Estevam,

quoted

On Fri, Aug 24, 2012 at 11:57 AM, Bastelli Carlo (yahoo)

CCing SPI list

quoted

[off-list ref] wrote:

quoted

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.

quoted

quoted

 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.

quoted

quoted

 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.

quoted

quoted

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?

quoted

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.

quoted

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