Re: [PATCH V4 2/3] dmaengine: tegra-adma: Add support for Tegra210 ADMA

From: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
To: Jon Hunter <jonathanh-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
Cc: Vinod Koul <vinod.koul-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>,
	Laxman Dewangan
	<ldewangan-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>,
	Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>,
	Thierry Reding
	<thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Alexandre Courbot
	<gnurou-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org>,
	Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>,
	Ian Campbell
	<ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>,
	Kumar Gala <galak-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>,
	dmaengine-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Subject: Re: [PATCH V4 2/3] dmaengine: tegra-adma: Add support for Tegra210 ADMA
Date: Tue, 19 Apr 2016 16:18:56 +0200	[thread overview]
Message-ID: <3969252.smixZttJLQ@wuerfel> (raw)
In-Reply-To: <571639D5.10108-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>

On Tuesday 19 April 2016 14:59:49 Jon Hunter wrote:
> On 19/04/16 14:25, Vinod Koul wrote:
> > On Mon, Apr 18, 2016 at 04:06:23PM +0100, Jon Hunter wrote:
> >> On 14/04/16 12:04, Jon Hunter wrote:
> >>> I had a look at this, but actually, I don't think this is going to work.
> >>>
> >>> Looking at dma_request_channel(), it is going to get a DMA channel that
> >>> matches the mask for any DMA controller. In the xlate I already know
> >>> which DMA controller I am and I just want one of the channels. The flow
> >>> here is ...
> >>>
> >>> dma_request_chan()
> >>>   --> of_dma_request_slave_channel()
> >>>     --> xlate()
> >>>       --> dma_get_any_slave_channel()
> >>>
> >>> There are several other DMA drivers that are calling
> >>> dma_get_any_slave_channel() from their xlate function which makes sense
> >>> because they are requesting one of their own channels.
> >>>
> >>> I can understand that you wish to consolidate the APIs for requesting a
> >>> channel, but it seems to me that you still need to have an API that DMA
> >>> controller drivers can call where they can pass their dma_device
> >>> structure to ensure you get a channel for the appropriate DMA controller.
> > 
> > Yes but the idea was that xlate will help you to get the right channel. The
> > whole dmaengine property was supposed to help you with that
> 
> Well it depends on the DMA controller. In the case of tegra the xlate
> helps you extract the slave request ID for a given device. However,
> because any channel can be used with any slave request ID, we don't care
> about the exact channel. So we request any available channel for the DMA
> controller in question and program it with the slave request we got from
> the xlate.

Right. the cleanup was supposed to reduce the number of interfaces
that a slave driver can call and consolidate them as much as possible
into dma_request_chan(), but we still need dma_get_any_slave_channel()
as an interface for the dmaengine masters as you said.

> I have seen some DMA drivers (such as imx-dma) use the filter function
> to match the DMA controller, but it seems daft to use the filter
> function to match the DMA controller when we already know in the xlate
> which DMA controller we are.

The filter functions should ideally all go away after the cleanup
is complete.

	Arnd