On Mon, May 18, 2015 at 10:06:55AM -0700, Dan Williams wrote: > 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/ It really depends on what you mean by "help". If you mean "undertake all by yourself the removal of async tx", then no, sorry, I won't, especially when you ask to do that for a patch that just enables a feature of an API already used on that platform. If you mean, "give me a hand, you can start there", then yeah, I can do that. > I'm not happy about not having had the time to do this rework myself. > Linux is better off with this api deprecated. You're not talking about deprecating it, you're talking about removing it entirely. Maxime -- Maxime Ripard, Free Electrons Embedded Linux, Kernel and Android engineering http://free-electrons.com
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