[lkml]   [2014]   [Mar]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] spi: core: make zero length transfer valid again
On 01.03.2014, at 05:13, Mark Brown <> wrote:
> On Fri, Feb 28, 2014 at 11:03:16PM +0900, Atsushi Nemoto wrote:
>> Zero length transfer becomes invalid since
>> "spi: core: Validate length of the transfers in message" commit,
>> but it should be valid to support an odd device, for example, which
>> requires long delay between chipselect and the first transfer, etc.

This "odd-device support" described sounds a like a work-arround
for missing functionality in spi_core.

Would it not be better to implement this as a separate member -
say: spi_transfer.pre_transfer_delay_usecs - and keep the
spi_transfer.len > 0 requirement?
Initially maybe make it a warning to find those odd-devices...

I am not sure if it might make some bus-drivers more complicated
/inefficient just to support this zero length.

For example: the spi-bcm2835.c driver would do the following with a
spi_transfer.len == 0 in the transfer_on method:
* enables SPI and wait for interrupt completion
* the above which will trigger an interrupt
** in the interrupt we find out that there is nothing to transfer,
so we signal completion to transfer_one, so it may continue.
* the main transfer_one will get woken up
** it will do a delay_usecs
** it will handle CS_CHANGE
** it will disable SPI/reset HW again

So this implementation shows that there is a lot of inefficient
overhead/delay just to trigger a delay...
This example requires 2 context switches (dwait for completion)
and its corresponding delays to get back to processing - so the
effective delay may be longer than 2ms just because of the delays
introduced via the scheduler and thus way above the delay requested
by the transfer...

OK - for the spi-bcm2835.c driver the following in
if (xfer->len == 0)
return 0;
would solve it, but then we might implement this:
if (xfer->pre_transfer_delay_usecs)
instead and be more explicit about this delay.

I guess other drivers will show similar code-artefacts and
some may even make the implicit assumption it has to be non-zero,
which would break functionality those odd devices.


 \ /
  Last update: 2014-03-01 14:21    [W:0.110 / U:0.060 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site