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SubjectProposed changes/improvements to SPI drivers
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



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