On 07/18/16 13:31, Russell King - ARM Linux wrote:

On Thu, Jul 14, 2016 at 03:42:35PM +0300, Peter Ujfalusi wrote:

quoted

Hi,

The following series with the final patch will add support for sDMA Linked List
transfer support.
Linked List is supported by sDMA in OMAP3630+ (OMAP4/5, dra7 family).
If the descriptor load feature is present we can create the descriptors for each
SG beforehand and let sDMA to walk them through.
This way the number of sDMA interrupts the kernel need to handle will drop
dramatically.

I suggested this a few years ago, and I was told by TI that there was
no interest to implement this feature as it had very little performance
effect.

I can not comment on this... Few years ago I was not involved with the DMA
drivers so I don't have any idea why would anyone object to have the linked
list (or descriptor load) mode in use whenever it is possible.
I was not even aware of the linked list mode of sDMA 3 weeks back, but while
reading the TRM - for the interleaved mode mainly it sounded like a good idea
to implement this.
Not really sure about the raw performance impact, but for interactivity it
does help. I remember running 'emerge --sync' on BeagleBoard was pain as it
took hours and the board was mostly unusable during that time. With the linked
list mode the same takes reasonable time and I can still poke around in the board.

Do I take it that TI have changed their position on this feature?

I was not aware of any position on this from TI - as I mentioned I was not
involved with DMA. It could be that the position from 'TI' is still what it
was. Or changed. But as I have been asked to look after TI DMA drivers
upstream and I believe that the linked list mode is a good thing to have -
which is backed by my experiences. My position is that linked list support is
cool.

-- 
P?ter