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

[Dan Williams]

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.
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