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

[Andy Shevchenko]

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.

-- 
With Best Regards,
Andy Shevchenko