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

Dan Williams
2009-09-15T17:28:48Z

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@intel.com>
>> ---
>
> Signed-off-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
>
> 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