[PATCH] spi: SPI_MASTER_MUST_* with scatter-gather only option and avoiding realloc

[Martin Sperl]

> On 22.05.2015, at 13:34, Mark Brown <broonie at kernel.org> wrote:
> 
> Quite a few process issues here which are rather annoying so I'm just
> going to ignore the technical content for now.
> 
Accepting those - thanks for the clarifications.

> 
>> If you can tell me that:
>> 	(master->flags & (SPI_MASTER_MUST_TX | SPI_MASTER_MUST_RX)) &&
>> 	master->can_dma && master->can_dma(...) 
>> means that we do NOT need real memory allocated in the first place but
>> we only expect a scatter list to be created 
>> and memory allocated in all other cases then that be it.
> 
> I replied to your last mail on this in the other thread.
Well - I will take that as a yes.

With the logic based only on can_dma we still would be allocating memory
for those cases where can_dma returns false - even if the driver only
requires the scatter/gather list but would not require memory for those
rx/tx_buf == NULL cases (which is the case for the spi-bcm2835).

The extra flags solve that requirement cleanly.

An alternative would be that we provide a spi_map_dummy function
that the driver could call explicitly if it just needs the scatter_gather
list for null-buffers and leave the master->flags = 0.

One other reasons was that with this logic we need to run can_dma several
times in the process at different stages which is a waste of CPU-resources
(especially as the compiler can not inline those function pointer calls).

To reduce that overhead, would it be acceptable if we add a field to
spi_transfer (and possibly to spi_message) - say bool can_dma -  that is
filled during the loop over all the transfers in __spi_validate?

Essentially adding something like this to __spi_validate:
	message->can_dma = false;
	list_for_each_entry(xfer, &message->transfers, transfer_list) {
		/* calculate can_dma once and cache it */
		xfer->can_dma = master->can_dma ? master->can_dma(xfer) : false;
		/* and set it also in the message itself */
		message->can_dma |= xfer->can_dma
	}