On Mon, Feb 04, 2013 at 09:47:38PM +0000, Arnd Bergmann wrote: > On Monday 04 February 2013, Linus Walleij wrote: > > So I think the above concerns are moot. The callback we can > > set on cookies is entirely optional, and it's even implemented by > > each DMA engine, and some may not even support it but require > > polling, and then it won't even be implemented by the driver. > > Just to ensure that everybody is talking about the same thing here: > Is it just the callback that is optional, or also the interrupt > coming from the hardware? If everyone implements stuff correctly, both. The callback most certainly is optional as things stand. The interrupt - that depends on the DMA engine. Some DMA engines you can't avoid it because you need to reprogram the hardware with the next+1 transfer upon completion of an existing transfer. Others may allow you to chain transfers in hardware. That's all up to how the DMA engine driver is implemented and how the hardware behaves. Now, there's another problem here: that is, people abuse the API. People don't pass DMA_CTRL_ACK | DMA_PREP_INTERRUPT into their operations by default. People like typing '0'. The intention of the "DMA_PREP_INTERRUPT" is significant here: it means "ask the hardware to send an interrupt upon completion of this transfer". Because soo many people like to type '0' instead in their DMA engine clients, it means that this flag is utterly useless today - you have to ignore it. So there's _no_ way for client drivers to actually tell the a DMA engine driver which _doesn't_ need to signal interrupts at the end of every transfer not to do so. So yes, the DMA engine API supports it. Whether the _implementations_ themselves do is very much hit and miss (and in reality is much more miss than hit.) -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
