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

Sosnowski, Maciej
2009-09-15T16:07:09Z

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?

Thanks,
Maciej