Re: Status of Linux SPI slave

[Ned Forrester <nforrester-/d+BM93fTQY@public.gmane.org>]

Roger Frøysaa wrote:
> Hi.
> 
> I'm wondering if there are any plans to add SPI slave mode to the SPI
>  driver framework in the "near" future? Is anyone working on
> something like this?
> 
> If not, has anyone ever considered it - and estimated the amount of
> work it would be etc?

I have been waiting to respond until there was a response from someone
knowledgeable.  As David points out, true slave capability has serious
real time implications, and the standard Linux kernel is not a real-time
kernel, as I understand it.  However, if you don't need completely
general slave capability, there may be ways to accomplish some
slave-like tasks.

For example, I have an SPI master device that streams data to a Linux
processor at 11Mbit/sec, but never expects any response from the
processor; this data is written over the 100baseT network to an NFS
file system at an effective 5GBytes/hour.  To make it work, I have
heavily modified the pxa2xx-spi driver for the target Gumstix system so
that it can accept data driven by an external clock, and so that it uses
a queue of DMA descriptors to prevent interrupt latency from limiting
service of the receive FIFO.  I did not have to modify a single line of
code anywhere else in the SPI framework.  All other parts of the SPI
framework, as well as my protocol driver and user-space code, still
operate as if they were part of an SPI master; they just keep full a
queue of SPI messages that request buffers to be filled by SPI reads.
Basically, this is a VERY limited slave function, where the Linux system
is not in control of the timing, but also there is never any sort of
query/response between the master and slave.

-- 
Ned Forrester                                       nforrester-/d+BM93fTQY@public.gmane.org
Oceanographic Systems Lab                                  508-289-2226
Applied Ocean Physics and Engineering Dept.
Woods Hole Oceanographic Institution          Woods Hole, MA 02543, USA
http://www.whoi.edu/sbl/liteSite.do?litesiteid=7212
http://www.whoi.edu/hpb/Site.do?id=1532
http://www.whoi.edu/page.do?pid=10079


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