RfC: Handle SPI controller limitations like maximum message length

[Martin Sperl]

> On 20.11.2015, at 12:06, Mark Brown <broonie at kernel.org> wrote:
> 
> On Thu, Nov 19, 2015 at 04:07:46PM -0800, Brian Norris wrote:
> 
>> So for transfer size limitations, are you speaking of the same thing as
>> Heiner (who began this post mentioning *message* size limitations)? I
>> read a difference between the two, where a transfer size limitation
>> might mean that one could improve the SPI core to just split transfers
>> up into multiple sub-transfers, and still complete the whole
>> spi_message (and therefore the protocol driver has less to worry about).
>> But if we're talking about spi_message limitations, then this would be
>> more exposed to the protocol driver.
> 
> For almost all hardware these should be the same things - most drivers
> shouldn't be working in terms of messages, they should be working in
> terms of transferrs.  Anything that has to work at full message level
> almost certainly has substantial other limitations in place given the
> need to be able to change parameters arbatrarily in the middle of
> messages.

I will try to get a prototype out soon, so that we can talk actual code,
but for now I will leave out the automated core logic for transformations.

Martin