On 06/09/2011 11:58 PM, Dan Williams wrote: > On Thu, Jun 9, 2011 at 2:38 AM, Linus Walleij <linus.walleij@xxxxxxxxxx> wrote: >> On Thu, Jun 9, 2011 at 8:54 AM, viresh kumar <viresh.kumar@xxxxxx> wrote: >> >>> I thought map_sg/unmap_sg for slave channels will be handled according >>> to the flags passed in prep_slave_sg(). But then i found following patch: >>> (...) >>> I don't have much knowledge about that discussion, but i think this should be left >>> configurable. >>> If the client wants to control map/unmap then it can simply pass >>> DMA_COMPL_SKIP_DEST_UNMAP | DMA_COMPL_SKIP_SRC_UNMAP in flags. I didn't wanted to >>> skip this in my driver and so i don't pass them. >> >> What if the same driver is used on many different platforms like say >> drivers/tty/serial/amba-pl011.c, and some of the platforms using it >> has DMA engines that does not implement mapping/unmapping of >> the passed sglist? >> >> In that case I think you have to modify all drivers in drivers/dma/* >> to do this mapping, and then you could just make it a required behaviour >> and skip the flags altogether. >> >> But apparently that approach was blocked at one point so let's see >> what the others say. > > My problem with automatic unmapping support is that the dma-driver > really does not have a chance to get it right except for the trivially > straightforward cases. One need only look at the current bustage of > raid5 acceleration with respect to overlapping mappings and arm v6. > The dma-driver just knows how to perform "this" operation on "this" > dma address. It does not know the lifetime of the mapping, or even if > it has the actual dma handle for unmapping versus an offset > > For the raid case I've currently convinced myself that the raid client > needs to get directly involved in dma mapping management, rather than > teach all dma drivers a language of how to unmap and when. Not only > will this fix the overlapping, but it also eliminates the need to map > and remap because the raid client knows the lifetime of a stripe_head > while the driver only knows the lifetime of a given stripe operation. > > For slave-dma maybe there is a lot of common un-mapping logic that can > be reused, but I think that comes from a separate smart library that > understands the dma mapping lifetimes of a given class of clients. > Leave the dma-drivers to just be dumb operators on anonymous dma > addresses. > Linus, Dan, Got it. Thanks for your replies. -- viresh -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
