Messages in this thread |  | | | Date | Tue, 6 Mar 2012 14:03:36 +0100 (CET) | | From | Guennadi Liakhovetski <> | | Subject | Re: [PATCH/RFC] dmaengine: add a slave parameter to __dma_request_channel() |
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On Tue, 6 Mar 2012, Vinod Koul wrote:
> On Tue, 2012-03-06 at 09:53 +0100, Guennadi Liakhovetski wrote: > > Hi Vinod > > > > Thanks for your review. > > > > On Tue, 6 Mar 2012, Vinod Koul wrote: > > > > > On Fri, 2012-03-02 at 14:21 +0100, Guennadi Liakhovetski wrote: > > > > Hi Vinod > > > > > > > > On Wed, 1 Feb 2012, Guennadi Liakhovetski wrote: > > > sorry I thought I had replied, but looks like it got missed! > > > > > > > > > When performing slame DMA some dmaengine drivers need additional data from > > > typo ^^^^^^^^^ > > > > > client drivers to find out, whether they can support that specific client > > > > > and to configure the DMA channel for it. This additional data has to be > > > > > supplied by client drivers during channel allocation, i.e., with the > > > > > __dma_request_channel() function. This patch adds a new > > > > > struct dma_slave_desc with some basic data in it, further this struct can > > > > > be embedded in hardware-specific types to supply any auxiliary > > > > > configuration. > > > counter arguing shouldn't the client drivers find out of the channel > > > requested is capable or not, that can be alternate approach as well. > > > That way people implement this in the filer functions and find if this > > > is the channel we need rather than dmac finding out if it can service > > > the client or not. > > > > How shall clients find this out? This is system- and DMAC-specific, this > > has nothing to do with the client functionality. The proposed approach is: > > > > * a client driver (MMC, USB, anything else) is capable to use DMA uses the > > standard dmaengine API to transfer the data > > > > * if the platform, where it's running, is supplying any auxiliary data, > > that it has to pass to the DMAC driver, it can do so, without getting > > involved in the details, just passing a pointer > > > > * the most natural location to do this is IMHO when requesting a DMA > > channel > and in that case why do you need the new parameters to be passed back in > filter function. What is the role of filter in this case ?
Sorry, who said I needed them? No changes are required to the filter function. As for its role - don't know, I didn't design it:-) In my case the filter will essentially become a "return true" dummy, respectively, it can simply be omitted. In general, I can hardly imagine a situation, where, say, an MMC driver would have internal knowledge about DMA channels on the system, allowing it to select a suitable one... So, I'm really not sure what it is for. Good, that it is at least optional. Maybe it can be deprecated with time.
> > Now, on sh-mobile platforms you can realistically have around 5 DMAC > > instances with 2 or 6 channels each, of which, say, 3 controllers are > > suitable for MMC and 2 are not. How shall the filter function find this > > out? Call some ugly platform callback? Traverse some platform-specific > > lists? Or use a fixed channel, thus significantly reducing flexibility? > > Sorry, none of these options seems very attractive to me. > well you can counter argue that dmac does not have this information > either.
But the DMAC is certainly a better match for making channel-selection decisions.
> Bigger question is who knows about this mapping and how do we > incorporate this mapping into channel allocation
The platform does. And this knowledge has to be passed to the relevant driver. But I think it's the DMAC driver, that is relevant, not the client driver. The platform would supply information like
DMAC #1 channel #1 (can be used for) device #1 device #2 ... channel #2 ... ...
And I don't think, it would be reasonable to let every slave driver use this information. These lists can also be optimised for specific platforms. E.g., on some sh-mobile SoCs you have two DMAC types. One of them can serve devices from list A on any channel, the other one - from list B. So, all you have to do, is to reference either A or B from your DMAC platform data. Whereas doing a reverse mapping: for each (potential) DMA user reference a list of channels, that it can use - would be really clumsy.
> > > Frankly I prefer former model, as that way dmacs will present channel > > > capabilities, and clients can use as they deem fit. > > > > > > > > > > Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/
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