Re: [PATCH/RFC] SPI: add DMAUNSAFE analog to David Brownell's core

David Brownell wrote:

No, "stupid drivers will suffer"; nothing new. Just observe
how the ads7846 touchscreen driver does small async transfers.


One cannot allocate memory in interrupt context, so the way to go is allocating it on stack, thus the buffer is not DMA-safe.
Making it DMA-safe in thread that does the very message processing is a good way of overcoming this.
Using preallocated buffer is not a good way, since it may well be already used by another interrupt or not yet processed by the worker thread (or tasklet, or whatever).
The way it's done in this ads7846 driver is not quite acceptable. Losing the transfer if the previous one is still processed is *not* the way to go in some cases. One can not predict how many transfers are gonna be dropped due to "previous trransfer is being processed" problem, it depends on the system load. And though it's not a problem for touchscreen, it *will* be a problem if it were MMC, for instance.

Vitaly
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