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

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Date:	Wed, 18 Nov 2015 16:41:35 +0200
From:	Peter Ujfalusi <peter.ujfalusi@...com>
To:	Arnd Bergmann <arnd@...db.de>
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 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...
> 
> 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.

-- 
Péter
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