linux-kernel - Re: [PATCH v2] DMA: add help function to check whether dma controller registered

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Message-ID: <52178628.5020702@wwwdotorg.org>
Date:	Fri, 23 Aug 2013 09:56:24 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Richard Zhao <rizhao@...dia.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"djbw@...com" <djbw@...com>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH v2] DMA: add help function to check whether dma controller
 registered

On 08/22/2013 07:17 PM, Richard Zhao wrote:
> On Fri, Aug 23, 2013 at 04:36:53AM +0800, Stephen Warren wrote:
>> On 08/22/2013 12:43 AM, Richard Zhao wrote:
>>> DMA client device driver usually needs to know at probe time whether
>>> dma controller has been registered to deffer probe. So add a help
>>> function of_dma_check_controller.
>>>
>>> DMA request channel functions can also used to check it, but they
>>> are usually called at open() time.
>>
>> This new function is almost identical to the existing
>> of_dma_request_slave_channel(). Surely the code should be shared?
>
> ofdma->of_dma_xlate(&dma_spec, ofdma);
> The above is called holding of_dma_lock. If I want to abstract the
> common lines, there' two options.

What is the problem with acquiring the lock? If request_slave_channel()
needs to take the lock, then there must be a reason which presumably
applies to the other path too.

...
>> But that said, I don't see any need for a new function; why can't
>> drivers simply call of_dma_request_slave_channel() at probe time;
>
> It'll mislead user. channel supposed to be request at open time.

I don't agree.

>> from
>> what I can see, that function doesn't actually request the channel, but
>> rather simply looks it up, just like this one. The only difference is
>> that of_dma_xlate() is also called, but that's just doing some data
>> transformation, not actually recording channel ownership.
>
> xlate function request the channel if things go well.

Oh. xlate should not do that; that's a design flaw. xlate should do
nothing more than translate the DT content to an internal channel ID.
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