On Fri, Jan 15, 2016 at 10:38 AM, Nicolin Chen nicoleotsuka@gmail.com wrote:
On Fri, Jan 15, 2016 at 09:03:28AM -0800, Caleb Crome wrote:
If nobody objects, we can just set the value to 4 and be done with it.
I agree. And we may apply it only to i.MX platforms with DMA if other platform owners feel comfortable with the previous settings.
Another question: is the watermark ever going to be different than maxburst? Is there any reason to have them different?
The watermark is merely a threshold to trigger a DMA request. The only relationship with the burst size is that each burst transfer should not carry more data than the number of empty slots; FIFO under/overflow occurs otherwise. So it's just more efficient and safer to set an identical value to both of them. I don't think it will cause functional problems to set TFWM to 4 and burst size to 1 -- It just lets DMA operate in a single data transfer mode.
If there is no penalty for setting maxburst to 1 (or 2 in the case of dual fifo I think), then should we just set both the watermark and maxburst to 1?
I guess the real difference would be when you're in FIQ mode. In FIQ mode, the penalty of an interrupt per word would be pretty bad, but in DMA mode, if we just set both to 1, we should be fine, right?