2011/3/22 James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>: > On Mon, 2011-03-21 at 22:03 +0100, Florian Mickler wrote: >> On Mon, 21 Mar 2011 15:26:43 -0400 >> Andy Walls <awalls@xxxxxxxxxxxxxxxx> wrote: >> >> > Florian Mickler <florian@xxxxxxxxxxx> wrote: >> >> To be blunt, I'm not shure I fully understand the requirements myself. >> But as far as I grasped it, the main problem is that we need memory >> which the processor can see as soon as the device has scribbled upon >> it. (think caches and the like) >> >> Somewhere down the line, the buffer to usb_control_msg get's to be >> a parameter to dma_map_single which is described as part of >> the DMA API in Documentation/DMA-API.txt >> >> The main point I filter out from that is that the memory has to begin >> exactly at a cache line boundary... > > The API will round up so that the correct region covers the API. > However, if you have other structures packed into the space (as very > often happens on stack), you get cache line interference in the CPU if > they get accessed: The act of accessing an adjacent object pulls in > cache above your object and destroys DMA coherence. This is the > principle reason why DMA to stack is a bad idea. Thanks, this was the missing piece of information to make sense of why it's bad for stack memory to be part of this. > >> I guess (not verified), that the dma api takes sufficient precautions >> to abort the dma transfer if a timeout happens. So freeing _should_ >> not be an issue. (At least, I would expect big fat warnings everywhere >> if that were the case) I did mean s/dma api/usb_control_msg/ in the above paragraph. As that is the ''dma api'' these drivers are using... sorry for the confusion there... > > No, it doesn't take any precautions like this. the DMA API is just > mapping (possibly via an IOMMU). If the transfer times out, that's done > in the DMA engine of the card, and must be cleaned up by the driver and > unmapped. ok. > The general rule though is never DMA to stack. On some processors, the > way stack is allocated can actually make this not work. > > James thanks, Flo p.s.: hope this message get's through to the list... I am on the road at the moment, so I'm not shure that there won't be any html in it again :( -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
