Proposed changes/improvements to SPI drivers

["Wojciech Zabołotny" <wzab@ise.pw.edu.pl>]

Hi,
I've worked with a SPI hardware requiring a little longer delay between 
deactivation and activation of CS line. To improve throughput, I had to 
perform transmission with a single spi_message consisting of
a few spi_transfers.
When debugging my driver, it appeared, that the usecs_delay allows me 
only to introduce delay
before CS gets deactivated. The delay between deactivation and 
activation of CS between transfers
is hardcoded:  (e.g. 
http://lxr.linux.no/linux+v2.6.37.1/drivers/spi/atmel_spi.c#L513 ).

I'd like to suggest adding another field (maybe "cs_inact_delay") like 
delay_usecs in the spi_transfer structure, which will define the time 
for which CS remains inactive:
(for example in atmel_spi it should look as follows:
  511   if (xfer->cs_change) {
  512          cs_deactivate(as, msg->spi);
  513          udelay(xfer->cs_inact_delay);
  514          cs_activate(as, msg->spi);
  515    }
)

Another problem, which I've faced was a little misleading description of 
the cs_change.
It has one additional functionality (at least in atmel_spi.c):
  When cs_change is set in the last transfer in a message, CS is not 
deactivated after the last transfer and is still active until the next 
spi_message is serviced (see 
http://lxr.linux.no/linux+v2.6.37.1/drivers/spi/atmel_spi.c#L395 , 
called from 
http://lxr.linux.no/linux+v2.6.37.1/drivers/spi/atmel_spi.c#L508 - if 
cs_change is non-zero, then "stay" is non-zero, and CS is not deactivated).

I don't know whether it is a bug or an undocumented feature...
-- 
Best Regards,
Wojciech Zabolotny