[linux-sunxi] Re: [PATCH] dma: sun4i: expose block size and wait cycle configuration to DMA users

[maxime.ripard@free-electrons.com (Maxime Ripard)]

On Tue, Mar 08, 2016 at 03:35:38PM +0530, Vinod Koul wrote:
> On Tue, Mar 08, 2016 at 09:42:31AM +0100, Hans de Goede wrote:
> > <wild speculation>
> > 
> > I see 2 possible reasons why waiting till checking for drq can help:
> > 
> > 1) A lot of devices have an internal fifo hooked up to a single mmio data
> > register which gets read using the general purpose dma-engine, it allows
> > this fifo to fill, and thus do burst transfers
> > (We've seen similar issues with the scanout engine for the display which
> >  has its own dma engine, and doing larger transfers helps a lot).
> > 
> > 2) Physical memory on the sunxi SoCs is (often) divided into banks
> > with a shared data / address bus doing bank-switches is expensive, so
> > this wait cycles may introduce latency which allows a user of another
> > bank to complete its RAM accesses before the dma engine forces a
> > bank switch, which ends up avoiding a lot of (interleaved) bank switches
> > while both try to access a different banj and thus waiting makes things
> > (much) faster in the end (again a known problem with the display
> > scanout engine).
> > 
> > </wild speculation>
> > 
> > Note the differences these kinda tweaks make can be quite dramatic,
> > when using a 1920x1080p60 hdmi output on the A10 SoC with a 16 bit
> > memory bus (real world worst case scenario), the memory bandwidth
> > left for userspace processes (measured through memset) almost doubles
> > from 48 MB/s to 85 MB/s, source:
> > http://ssvb.github.io/2014/11/11/revisiting-fullhd-x11-desktop-performance-of-the-allwinner-a10.html
> > 
> > TL;DR: Waiting before starting DMA allows for doing larger burst
> > transfers which ends up making things more efficient.
> > 
> > Given this, I really expect there to be other dma-engines which
> > have some option to wait a bit before starting/unpausing a transfer
> > instead of starting it as soon as (more) data is available, so I think
> > this would make a good addition to dma_slave_config.
> 
> I tend to agree but before we do that I would like this hypothesis to be
> confirmed :)

We can't confirm it, we don't have access to any documentation that
might explain what this is about.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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