[RFC] SPI transfer with (count % bpw >> 3) != 0

[RFC] SPI transfer with (count % bpw >> 3) != 0

[Sebastian Andrzej Siewior]

Just stumbled over this one: for instance bpw = 16 and len = 1.
I've found a few possible results:
- no transfer at all and return 0 (spi_gpio)
- no transfer at all and return with -EINVAL (spi_mpc8xxx)
- transfer two bytes and overwrite one probably unallocated byte in the
  read case. Return 1 (amba-pl022)
- write until an exception stops it (orion_spi)

Variant three and four is obviously wrong and should be fixed. However
I'm torn between variant one and two as the correct way to deal with
this. This problem is not just an in-kernel thing but also exposed to
user space via spidev. The man page for read() and write() says that the
function might return less than the number of requested bytes if more
data is not available. In this case there is more data available but
due to an programming error (?) we don't get anything. From that angle
-EINVAL sounds good.
David?

Sebastian

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