Re: spi transfer with zero length
From: David Brownell
Date: Sat Feb 16 2008 - 13:59:10 EST
On Saturday 16 February 2008, Atsushi Nemoto wrote:
> Hi. Is it legal to use zero for 'len' field of struct spi_transfer?
> I mean, len=0, tx_buf=rx_buf=NULL, delay_usecs!=0.
Yes that should work ... it's uncommon, but not illegal. Some
controller drivers may even handle that right!
If the delay were zero and cs_change didn't indicate a need to
briefly deselect the chip, it might make sense to reject such
a NOP transfer. But that's not the case you identify.
> Some SPI devices need slightly long delay before first CLK edge after
> CS assertion.
For future reference ... could you identify a few such devices,
and say what "long" is relative to the clock period?
Some folk have just slowed down the clock in such cases, but
that's rather sub-optimal.
> To achieve this, I think inserting using a zero length
> transfer before real transfers. But it seems some drivers do not
> handle this case properly.
Feel free to submit patches fixing those bugs.
> Is this driver's bug, or we need additional delay field in struct
> spi_device for such case?
I'd like to avoid new parameters to cover case that can already
be expressed in the programming interface. Cases that can't be
expressed ... different issue. I suspect any patches updating
timing parameters should use nanoseconds not microseconds, fwiw.
- Dave
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