On Mon, May 18, 2015 at 2:14 AM, Maxime Ripard <maxime.ripard@xxxxxxxxxxxxxxxxxx> wrote: > Hi Dan, > > On Wed, May 13, 2015 at 09:00:46AM -0700, Dan Williams wrote: >> 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. > > I'm not sure what the issues exactly are with async_tx and ARM, but > these patches have been tested on ARM and are working quite well. https://lkml.org/lkml/2011/7/8/363 > What I'm doing here is merely using the existing API, I'm not making > it worse, just using the API that is used by numerous drivers > already. So I'm not sure this is really reasonable to ask for such a > huge rework (with a huge potential of regressions) before merging my > patches. It happens. https://lwn.net/Articles/641443/ I'm not happy about not having had the time to do this rework myself. Linux is better off with this api deprecated. -- 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
