RE: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch"allocator
From: Sosnowski, Maciej
Date: Tue Sep 15 2009 - 12:07:17 EST
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
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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
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validate, pq, pq validate, and memcpy. When
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ASYNC_TX_DISABLE_CHANNEL_SWITCH=y any channel with all these
>
capabilities is marked DMA_ASYNC_TX allowing async_tx_find_channel() to
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quickly locate compatible channels with the guarantee that dependency
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chains will remain on one channel. When
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ASYNC_TX_DISABLE_CHANNEL_SWITCH=n async_tx_find_channel() may select
>
channels that lead to operation chains that need to cross channel
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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:
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+ /* note: this only matters in the
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+ * CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH=y case
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+ */
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+ if (device_has_all_tx_types(device))
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+ 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--
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