On Wed, May 13, 2015 at 2:17 AM, Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > Hi Dan, > > On Tue, May 12, 2015 at 09:05:41AM -0700, Dan Williams wrote: >> On Tue, May 12, 2015 at 8:37 AM, Maxime Ripard >> <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: >> > Hi, >> > >> > This serie refactors the mv_xor in order to support the latest Armada >> > 38x features, including the PQ support in order to offload the RAID6 >> > PQ operations. >> > >> > Not all the PQ operations are supported by the XOR engine, so we had >> > to introduce new async_tx flags in the process to identify >> > un-supported operations. >> > >> > Please note that this is currently not usable because of a possible >> > regression in the RAID stack in 4.1 that is being discussed at the >> > moment here: https://lkml.org/lkml/2015/5/7/527 >> >> This is problematic as async_tx is a wart on the dmaengine subsystem >> and needs to be deprecated, I just have yet to find the time to do >> that work. It turns out it was a mistake to hide the device details >> from md, it should be explicitly managing the dma channels, not >> relying on a abstraction api. The async_tx api usage of the >> dma-mapping api is broken in that it relies on overlapping mappings of >> the same address. This happens to work on x86, but on arm it needs >> explicit non-overlapping mappings. I started the work to reference >> count dma-mappings in 3.13, and we need to teach md to use >> dmaengine_unmap_data explicitly. Yielding dma channel management to >> md also results in a more efficient implementation as we can dma_map() >> the stripe cache once rather than per-io. The "async_tx_ack()" >> disaster can also go away when md is explicitly handling channel >> switching. > > Even though I'd be very much in favor of deprecating / removing > async_tx, is it something likely to happen soon? Not unless someone else takes it on, I'm actively asking for help. > I remember discussing this with Vinod at Plumbers back in October, but > haven't seen anything since then. Right, "help!" :) > If not, I think that we shouldn't really hold back patches to > async_tx, even though we know than in a year from now, it's going to > be gone. We definitely should block new usages, because they make a bad situation worse. Russell already warned that the dma_mapping api abuse could lead to data corruption on ARM (speculative pre-fetching). We need to mark ASYNC_TX_DMA as "depends on !ARM" or even "depends on BROKEN" until we can get this resolved. -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
