Re: [PATCH v2 3/6] dmaengine: dw: Set DMA device max segment size parameter

[Serge Semin <Sergey.Semin@baikalelectronics.ru>]

On Tue, May 12, 2020 at 03:35:51PM +0300, Andy Shevchenko wrote:
> On Tue, May 12, 2020 at 12:16:22AM +0300, Serge Semin wrote:
> > On Fri, May 08, 2020 at 02:21:52PM +0300, Andy Shevchenko wrote:
> > > On Fri, May 08, 2020 at 01:53:01PM +0300, Serge Semin wrote:
> > > > Maximum block size DW DMAC configuration corresponds to the max segment
> > > > size DMA parameter in the DMA core subsystem notation. Lets set it with a
> > > > value specific to the probed DW DMA controller. It shall help the DMA
> > > > clients to create size-optimized SG-list items for the controller. This in
> > > > turn will cause less dw_desc allocations, less LLP reinitializations,
> > > > better DMA device performance.
> 
> > > Yeah, I have locally something like this and I didn't dare to upstream because
> > > there is an issue. We have this information per DMA controller, while we
> > > actually need this on per DMA channel basis.
> > > 
> > > Above will work only for synthesized DMA with all channels having same block
> > > size. That's why above conditional is not needed anyway.
> > 
> > Hm, I don't really see why the conditional isn't needed and this won't work. As
> > you can see in the loop above Initially I find a maximum of all channels maximum
> > block sizes and use it then as a max segment size parameter for the whole device.
> > If the DW DMA controller has the same max block size of all channels, then it
> > will be found. If the channels've been synthesized with different block sizes,
> > then the optimization will work for the one with greatest block size. The SG
> > list entries of the channels with lesser max block size will be split up
> > by the DW DMAC driver, which would have been done anyway without
> > max_segment_size being set. Here we at least provide the optimization for the
> > channels with greatest max block size.
> > 
> > I do understand that it would be good to have this parameter setup on per generic
> > DMA channel descriptor basis. But DMA core and device descriptor doesn't provide
> > such facility, so setting at least some justified value is a good idea.
> > 
> > > 
> > > OTOH, I never saw the DesignWare DMA to be synthesized differently (I remember
> > > that Intel Medfield has interesting settings, but I don't remember if DMA
> > > channels are different inside the same controller).
> > > 
> > > Vineet, do you have any information that Synopsys customers synthesized DMA
> > > controllers with different channel characteristics inside one DMA IP?
> > 
> > AFAICS the DW DMAC channels can be synthesized with different max block size.
> > The IP core supports such configuration. So we can't assume that such DMAC
> > release can't be found in a real hardware just because we've never seen one.
> > No matter what Vineet will have to say in response to your question.
> 
> My point here that we probably can avoid complications till we have real
> hardware where it's different. As I said I don't remember a such, except
> *maybe* Intel Medfield, which is quite outdated and not supported for wider
> audience anyway.

I see your point. My position is different in this matter and explained in the
previous emails. Let's see what Viresh and Vinod think of it.

-Sergey

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>