Messages in this thread | | | Date | Mon, 08 Jul 2013 16:28:29 +0200 | From | Lars-Peter Clausen <> | Subject | Re: [RFC] dmaengine: add dma_slave_get_caps api |
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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.
E.g. if the client has an internal FIFO with room for 16 samples and it sends a request once the FIFO is half empty, max_burst would be set to 8. If the master supports burst sizes of 8 sample it would send 8 samples, if it only supported burst sizes of 4 it might send either one burst of 4 or two of 4.
- Lars
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