Re: [PATCH 0/8] ARM: mvebu: Add support for RAID6 PQ offloading

[Maxime Ripard]

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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