linux-kernel - Re: [PATCH 02/13] dmaengine: Introduce dma_request_slave_channel_compat

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Date:	Wed, 18 Nov 2015 16:07:42 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Peter Ujfalusi <peter.ujfalusi@...com>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tony Lindgren <tony@...mide.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org,
	"linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	Linux MMC List <linux-mmc@...r.kernel.org>,
	linux-crypto@...r.kernel.org,
	linux-spi <linux-spi@...r.kernel.org>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	ALSA Development Mailing List <alsa-devel@...a-project.org>
Subject: Re: [PATCH 02/13] dmaengine: Introduce dma_request_slave_channel_compat_reason()

On Wednesday 18 November 2015 16:41:35 Peter Ujfalusi wrote:
> On 11/18/2015 04:29 PM, Arnd Bergmann wrote:
> > On Wednesday 18 November 2015 16:21:26 Peter Ujfalusi wrote:
> >> 2. non slave channel requests, where only the functionality matters, like
> >> memcpy, interleaved, memset, etc.
> >> We could have a simple:
> >> dma_request_channel(mask);
> >>
> >> But looking at the drivers using dmaengine legacy dma_request_channel() API:
> >> Some sets DMA_INTERRUPT or DMA_PRIVATE or DMA_SG along with DMA_SLAVE:
> >> drivers/misc/carma/carma-fpga.c                 DMA_INTERRUPT|DMA_SLAVE|DMA_SG
> >> drivers/misc/carma/carma-fpga-program.c         DMA_MEMCPY|DMA_SLAVE|DMA_SG
> >> drivers/media/platform/soc_camera/mx3_camera.c  DMA_SLAVE|DMA_PRIVATE
> >> sound/soc/intel/common/sst-firmware.c           DMA_SLAVE|DMA_MEMCPY
> >>
> >> as examples.
> >> Not sure how valid are these...

I just had a look myself. carma has been removed fortunately in linux-next,
so we don't have to worry about that any more.

I assume that the sst-firmware.c case is a mistake, it should just use a
plain DMA_SLAVE and not DMA_MEMCPY.

Aside from these, everyone else uses either DMA_CYCLIC in addition to
DMA_SLAVE, which seems valid, or they use DMA_PRIVATE, which I think is
redundant in slave drivers and can be removed.

> > It's usually not much harder to separate out the legacy case from
> > the normal dma_request_slave_channel_reason(), so those drivers don't
> > really need to use the unified compat API.
> 
> The current dma_request_slave_channel()/_reason() is not the 'legacy' API.
> Currently there is no way to get the reason why the dma channel request fails
> when using the _compat() version of the API, which is used by drivers which
> can be used in DT or in legacy mode as well. Sure, they all could have local
> if(){}else{} for handling this, but it is not a nice thing.
> 
> As it was discussed instead of adding the _reason() version for the _compat
> call, we should simplify the dmaengine API for getting the channel and at the
> same time we will have ERR_PTR returned instead of NULL.

What I meant was that we don't need to handle them with the unified
simple interface. The users of DMA_CYCLIC can just keep using
an internal helper that only deals with the legacy case, or use
dma_request_slave() or whatever is the new API for the DT case.

	Arnd
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