linux-kernel - Re: [PATCH v2 05/14] arm: common: edma: Select event queue 1 as default when booted with DT

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Message-ID: <20140411124641.GC32284@intel.com>
Date:	Fri, 11 Apr 2014 18:16:41 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
Cc:	Sekhar Nori <nsekhar@...com>, dan.j.williams@...el.com,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	joelf@...com, linux-arm-kernel@...ts.infradead.org,
	linux-omap@...r.kernel.org,
	davinci-linux-open-source@...ux.davincidsp.com, mporter@...aro.org,
	Mark Brown <broonie@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Liam Girdwood <lgirdwood@...il.com>,
	Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH v2 05/14] arm: common: edma: Select event queue 1 as
 default when booted with DT

On Fri, Apr 11, 2014 at 03:23:54PM +0300, Peter Ujfalusi wrote:
> On 04/11/2014 02:31 PM, Vinod Koul wrote:
> 
> >> I would say that it is channel based config. I don't see the reason why would
> >> one mix different priorities on a configured channel between descriptors.
> >>
> >>> If not then we can add this in dma_slave_config ?
> >>
> >> So adding to the struct for example:
> >> bool high_priority;
> > 
> > In designware controller, we can have channel priorties from 0 to 7 which IIRC 7
> > being highest. So bool wont work. unsigned int/u8 would be good.
> 
> I see. But from a generic code what number should one pass if we want to get
> the highest priority? With eDMA3 we essentially have three levels (see later)
> so if we receive 7 as priority what shall we do? Just treat as highest? But if
> we receive 4, which is somewhere in the middle for designware it is still
> means highest for us.
> 
> I see this too small step partitioning in common code a bit problematic when
> it comes to how to interpret the 'magic numbers'.
> Also how all the driver in the system will decide the priority number? I'm
> sure they will pick something conveniently average for themselves and I
> imagine audio would try to pick something which is bigger than the numbers
> others have taken...
> 
> > How about your controller, is it binary?
> 
> We also have priority from 0 to 7, 0 being the highest priority. We have 3
> Transfer Controllers/Event Queues and we can set the priority for the TC/EQ
> and assign the given channel to the desired TC/EQ.
> So in reality we have 3 priorities to choose from in my view since we only
> have 3 TC/EQ in eDMA3 (of AM335x/AM447x) on other SoCs we can have different
> number of TC/EQ.

I think the number shouldn't be viewed in absolute terms. If we decide that (lets
say) 0-7, then any controller should map 0 to lowest and 7 to highest.

For your case you can do this and then intermediate numbers would be medium
priority. Such a system might work well...

Also how would a client driver know which priority to use? Would it come from
DT?

-- 
~Vinod
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