Messages in this thread | | | Date | Wed, 30 Jan 2008 10:39:47 -0700 | From | "Dan Williams" <> | Subject | Re: [RFC v2 0/5] dmaengine: Slave DMA interface and example users |
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On Jan 30, 2008 1:56 AM, Haavard Skinnemoen <hskinnemoen@atmel.com> wrote: > On Tue, 29 Jan 2008 22:56:14 -0800 > David Brownell <david-b@pacbell.net> wrote: > > > On Tuesday 29 January 2008, Haavard Skinnemoen wrote: > > > > > > Btw, there's one issue I forgot to mention: I believe the DMA Engine > > > framework is currently misusing the DMA mapping API, and this patchset > > > makes things worse. > > > > > > Currently, the async_tx bits of the API do the required calls to > > > dma_map_single() and/or dma_map_page(), but they rely on the driver to > > > do the unmapping. This is problematic ... > > > > > > How do we solve this? > > > > How about: for peripheral DMA, don't let the engine see anything > > except dma_addr_t values. > > I don't think it does, but the dma_addr_t value is enough to call > dma_unmap_single() and dma_unmap_page().
Right, dma_addr_t values are all the driver sees in the current scheme.
> > > The engine needs to be able to dma_alloc_coherent() memory too, > > which is pre-mapped. > > Right, which is another argument for not doing any unmapping in the DMA > engine driver. We really need to push this responsibility to the client. >
Agreed, the issue is how to do this without requiring an interrupt+callback sequence for each transaction or requiring the client to carry per transaction unmap-data. For example NET_DMA never sees a dma_addr_t and assumes that all it needs to care about is the last transaction in a sequence. Since it is alive for the duration of a transaction, we could put unmap data in dma_async_tx_descriptor along with an unmap function pointer since dma_unmap* routines have an equal number of parameters. But I just got through making this structure smaller so maybe there is a better way.
-- Dan
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