Re: [spi-devel-general] [PATCH] atmel_spi: support zero length transfer

[David Brownell <david-b@pacbell.net>]

> > If the driver could not handle zero length transfer, then the driver
> > should reject it (just like unsupported transfer mode).  Then the
> > behavior will be 'assert chip select and wait some time' or 'rejected
> > by the driver'.
>
> This would be OK.  It would not be hard to fix pxa2xx_spi, for example,
> to reject zero-length transfers in DMA mode, as long as it is acceptable
> to reject the message in mid-message.

Such "illegal message" rejection is best done early; "fail-fast".
Mid-message rejection isn't wrong, but it's a rude policy.

It'd be best to fix pxa2xx_spi to not start zero-length DMAs.


>	If it were necessary to scan a
> whole message for zero-length transfers and refuse to queue an offending
> message, then that adds burden to all messages.

Sanity checking messages as they're submitted is easy; and it's
also the natural point for setting up DMA mappings (and making
those cachelines available for better use).

- Dave