Messages in this thread | | | Date | Tue, 15 Sep 2009 10:28:46 -0700 | From | Dan Williams <> | Subject | Re: [PATCH 02/18] dmaengine, async_tx: add a "no channel switch" allocator |
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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
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