spi: atmel: Prevent false timeouts on long transfers

spi: atmel: Prevent false timeouts on long transfers

A slow SPI bus clocks at ~20MHz, which means it would transfer about
2500 bytes per second with a single data line. Big transfers, like when
dealing with flashes can easily reach a few MiB. The current DMA timeout
is set to 1 second, which means any working transfer of about 4MiB will
always be cancelled.

With the above derivations, on a slow bus, we can assume every byte will
take at most 0.4ms. Said otherwise, we could add 4ms to the 1-second
timeout delay every 10kiB. On a 4MiB transfer, it would bring the
timeout delay up to 2.6s which still seems rather acceptable for a
timeout.

The consequence of this is that long transfers might be allowed, which
hence requires the need to interrupt the transfer if wanted by the
user. We can hence switch to the _interruptible variant of
wait_for_completion. This leads to a little bit more handling to also
handle the interrupted case but looks really acceptable overall.

While at it, we drop the useless, noisy and redundant WARN_ON() call.

Signed-off-by: Miquel Raynal <[email protected]>
Acked-by: Ryan Wanner <[email protected]>
Link: https://lore.kernel.org/r/Message-Id:
Signed-off-by: Mark Brown <[email protected]>