Re: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch"allocator

[Dan Williams]

Sosnowski, Maciej wrote:
> Williams, Dan J wrote:
> > Channel switching is problematic for some dmaengine drivers as the
> > architecture precludes separating the ->prep from ->submit.  In these
> > cases the driver can select ASYNC_TX_DISABLE_CHANNEL_SWITCH to modify
> > the async_tx allocator to only return channels that support all of the
> > required asynchronous operations.
> >
> > For example MD_RAID456=y selects support for asynchronous xor, xor
> > validate, pq, pq validate, and memcpy.  When
> > ASYNC_TX_DISABLE_CHANNEL_SWITCH=y any channel with all these
> > capabilities is marked DMA_ASYNC_TX allowing async_tx_find_channel() to
> > quickly locate compatible channels with the guarantee that dependency
> > chains will remain on one channel.  When
> > ASYNC_TX_DISABLE_CHANNEL_SWITCH=n async_tx_find_channel() may select
> > channels that lead to operation chains that need to cross channel
> > boundaries using the async_tx channel switch capability.
> >
> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > ---
>
> Signed-off-by: Maciej Sosnowski <maciej.sosnowski@xxxxxxxxx>
>
> with one question:
>
> > +	/* note: this only matters in the
> > +	 * CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH=y case
> > +	 */
> > +	if (device_has_all_tx_types(device))
> > +		dma_cap_set(DMA_ASYNC_TX, device->cap_mask);
>
> Why not to put this part plus device_has_all_tx_types()
> into #ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH then?

Because device_has_all_tx_types() already has the necessary ifdefs per 
operation type, and it does not rule out the possibility that code 
outside of async_tx can make use of DMA_ASYNC_TX capability.  We don't 
gain anything by eliminating this call... although we would save an 
entry in the dmaengine channel table by turning off DMA_ASYNC_TX in enum 
dma_transaction_type.  But we have already cleaned up 
dma_transaction_type in this patchset so the net change from 2.6.31 to 
2.6.32 is still positive in this area.

--
Dan
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