Re: [PATCH] dmaengine: at_hdmac: add DMA slave transfers
From: Dan Williams
Date: Wed Jul 29 2009 - 10:28:11 EST
Atsushi Nemoto wrote:
On Mon, 27 Jul 2009 14:24:26 +0100, "Sosnowski, Maciej" <maciej.sosnowski@xxxxxxxxx> wrote:
Your atc_chain_complete() calls dma_unmap_xxx unless
DMA_COMPL_SKIP_XXX_UNMAP specified. But atmel-mci driver does not set
the flag on dma_async_tx_descriptor. I suppose one of them should be
fixed.
atmel-mci should definitely set that flag.
Haavard
I agree with Haavard.
Then, what should dma driver do when client driver did not set these
flags? If it should call dma_unmap_sg(), the dma driver should keep
sg and direction somewhere...
Also, calling dma_map_sg() in its prep_slave_sg function will not fit
for sound drivers, which use DMA buffers prepared in its framework.
For slave DMA, doing all mapping/unmapping in DMA client is better,
isn't it?
Yes it is. The whole point of the dma driver doing its own unmapping is
specifically for clients that use the async_tx api, i.e. where they do
not know if the transaction is carried out in hardware and have no way
of tracking the details necessary to perform the unmap. DMA-slave
clients request specific channels and know the hardware details at a low
level, so it should not be too high an expectation to push dma mapping
responsibility to the client.
--
Dan
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