Re: [Question] PM-QoS: PM_QOS_CPU_DMA_LATENCY == interrupt latency?

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> > No. Well it may be on some platforms but it isn't the same thing. On some
> > devices a DMA transfer doesn't need the CPU involved but needs the CPU to
> > respond within a set timescale (eg for coherency or bus arbitration). It
> 
> I understand only the CPU can respond after it is notified by a
> interrupt event, don't I?

The instruction stream being executed maybe, but not things like the cache

> Also could you give a example about how the CPU responds to a DMA transfer
> within a set timescale if it is required?

The kind of thing you are dealing with is

	DMA engine requests a cache line of data
	CPU wakes out of sleep, completes bus transaction
	CPU goes back to sleep
	DMA engine starts outputting data bits over SPI bus or similar

	repeat until done

so it's not instruction level stuff, merely bus traffic.