Messages in this thread | | | Date | Mon, 08 Jul 2013 16:54:59 +0200 | From | Lars-Peter Clausen <> | Subject | Re: [RFC] dmaengine: add dma_slave_get_caps api |
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On 07/08/2013 04:03 PM, Vinod Koul wrote: > On Mon, Jul 08, 2013 at 04:28:29PM +0200, Lars-Peter Clausen wrote: >> On 07/08/2013 03:40 PM, Vinod Koul wrote: >>> On Mon, Jul 08, 2013 at 02:01:35PM +0200, Lars-Peter Clausen wrote: >>>> On 07/08/2013 10:54 AM, Vinod Koul wrote: >>>>> +/* struct dma_slave_caps - expose capabilities of a slave channel only >>>>> + * >>>>> + * @src_addr_widths: bit mask of src addr widths the channel supports >>>>> + * @dstn_addr_widths: bit mask of dstn addr widths the channel supports >>>>> + * @src_burst_lengths: bit mask of src slave burst lengths supported >>>>> + * @dstn_burst_lengths: bit mask of dstn slave burst lengths supported >>>> >>>> I'm not sure if we need the burst_lengths fields. For one we can only >>>> express a max burst length up 32. And usually it is fine if the dma >>>> controller does not support the burst length requested by the slave driver, >>>> since this only specifies the maximum and the dma controller driver can >>>> choose a value below this limit. E.g. if the max burst length is set to 16 >>>> it is still fine if the controller only supports a burst length of 8. >>> well how are you picking up which one to use? >>> The idea is that you query and match that with system and client to get best >>> throughput. If you have IP block which over generations change capablities you >>> can runtime query and then program the channel smartly! >> >> The client would always request the largest burst size it can support, and >> the dma master would then select a burst size equal or smaller to it >> depending on what it can support. > not really, most dmac drivers will return an error when they find a value is not > in the supported list. And that is precisely what we would like to prevent. > If you look at current model, it mostly program value which we know is already > a capablity and works model.
The majority of the DMA drivers will either accept any maxburst value (which is probably broken) or round down. The only ones which accept only a fixed set of burst sizes seem to be mmp_pdma, mmp_tdma and sa11x0-dma.
- Lars
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