Slow spi_sync() on pxa2xx_spi

[Stefan Schmidt <stefan@datenfreihafen.org>]

Hello.

I'm working on the Imote2 (pxa27x based) platform [1] which includes a
radio chip (cc2420, IEEE 802.15.4) connected over SPI [2].

The problem I'm facing is that the spi_sync() call of the driver to
write/read from the registers over the SPI bus takes up to 26 milli
seconds. The minimum I measured was 500 micro seconds for the call.

That creates a problem as the workflow to send a frame over the radio
includes to write to a spi register for sending out the frame from a
pre-filled FIFO. Then wait up to 320 micro seconds for a GPIO to raise
and polling a status register until a TX_ACTIVE bit is no longer set
afterwards.

But the first register write to initiate the sending already takes so
long that the GPIO is already low again and the TX_ACTIVE is also gone
when I come back from the spi_sync() call.

It really smells like I'm doing something seriously wrong here. For
the pxa2xx_spi driver I already tried various options without any
difference I could see. Enable/disbale DMA, different tx/rx
thresholds, different dma burst sizes, different timeout, etc.

I can only explain the long time with some sleeping in SPI while
other parts are scheduled. Is there a way to avoid this for SPI? Or
is there an API I did not found yet to cover my needs?

The measurement is done with getnstimeofday btw and I verified that a
basic operation like two multiplications are down in the 5 micro
seconds area which gives me faith that the measurements should be
correct:

getnstimeofday (&before);
ret = spi_sync(lp->spi, &msg);
getnstimeofday (&after);

result = timespec_sub(after, before);
printk("Strobe time: %lu secs and %lu nsecs (strobe %i)\n", (long)result.tv_sec, result.tv_nsec, addr);

I'm out of ideas what to change here. The only option, I want to
avoid, is trying to drive the SPI pins manually and see if I have
better results with bit-banging it myself.

regards
Stefan Schmidt

[1] Board: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/arm/mach-pxa/stargate2.c
[2] Driver:
http://linux-zigbee.git.sourceforge.net/git/gitweb.cgi?p=linux-zigbee/kernel;a=blob;f=drivers/ieee802154/cc2420.c;h=50761de6eb2d87014b6e43daa4ed642319d10567;hb=6a1a3375886ba69b1969ceb5df3007a4f1791d27