Re: async_tx and RAID HW xor engine

[NeilBrown <neilb@suse.de>]

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On Mon, 30 Apr 2012 13:31:41 -0700 Dan Williams <dan.j.williams@intel.com>
wrote:

> [ adding Russell and Neil for comment about the proposed change to async_tx ]

I've managed to avoid learning much about the internal details of async_tx
and adma but I thought I'd have a quick look.

Doesn't this Kconfig change simply disable ASYNC_TX_DMA on any device that
selects ASNC_TX_ENABLE_CHANNEL_SWITCH.  i.e.
  INTEL_IOP_ADMA
  FSL_DMA
  MX_XOR
  AMCC_PPC440_SPA_ADMA
Is that what you want to do?  Disable all of those?
I guess if they don't work then that is the right thing to do....

NeilBrown




> 
> On Mon, Apr 30, 2012 at 3:05 AM, Rajasekhar Pulluru
> <pullururajasekhar@gmail.com> wrote:
> > Hi,
> >
> > I am trying to understand how are hardware xor engine's utilized by
> > async_tx module. I don't get the exact link that connects async_tx and
> > hardware xor engine.
> > Could someone help me understand it.
> >
> > Registration of a RAID HW XOR engine happens in
> > drivers/dma/<soc-xor.c> using dma_async_device_register(). Does this
> > registration function connect the async_tx api's
> > to use the HW XOR engine?
> 
> Yes, the channel registers its capabilities with dmaengine, and then
> async_tx asks dmaengine (via dma_find_channel) for a resource to carry
> out the operation(s).
> 
> > Or is there anything we need to support to
> > make async_tx api's to take advantage of the hardware XOR engine?
> 
> Registration with dma engine should be all that is needed, but a word
> of caution that the channel switching mechanism has been found to be
> incompatible with the dma mapping api.  So, unless your dma channel
> supports all the operations needed for raid, do not rely on async_tx's
> channel switching capabilities.
> 
> An architecture where this is known to be a problem is ARM, but I
> wonder if we should take the following global step until this is fixed
> properly?
> 
> diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
> index cf9da36..b812b6b 100644
> --- a/drivers/dma/Kconfig
> +++ b/drivers/dma/Kconfig
> @@ -280,6 +280,7 @@ config NET_DMA
>  config ASYNC_TX_DMA
>         bool "Async_tx: Offload support for the async_tx api"
>         depends on DMA_ENGINE
> +       depends on !ASYNC_TX_ENABLE_CHANNEL_SWITCH
>         help
>           This allows the async_tx api to take advantage of offload engines for
>           memcpy, memset, xor, and raid6 p+q operations.  If your platform has


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