linux-kernel - Re: [PATCHv2] DMAEngine: Let dmac drivers to set chan

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Date:	Tue, 26 Jul 2011 20:00:16 +0530
From:	Jaswinder Singh <jaswinder.singh@...aro.org>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	linux-kernel@...r.kernel.org, vinod.koul@...el.com,
	rmk+kernel@....linux.org.uk, linus.walleij@...ricsson.com,
	per.friden@...ricsson.com, wei.zhang@...escale.com,
	ebony.zhu@...escale.com, iws@...o.caltech.edu,
	s.hauer@...gutronix.de, maciej.sosnowski@...el.com,
	saeed@...vell.com, shawn.guo@...escale.com, yur@...raft.com,
	agust@...x.de, iwamatsu.nobuhiro@...esas.com,
	per.forlin@...ricsson.com, jonas.aberg@...ricsson.com,
	anemo@....ocn.ne.jp
Subject: Re: [PATCHv2] DMAEngine: Let dmac drivers to set chan_id

On 26 July 2011 01:38, Williams, Dan J <dan.j.williams@...el.com> wrote:

>>> I can sort of see why it is attractive for the goal of having clients
>>> being able to talk to multiple DMACs since it is a field that all
>>> channels already implement.  But, I don't think we should be mixing
>>> sysfs presentation details with a capability that indicates a certain
>>> compatibility level in a DMAC driver implementation.  If the goal is
>>> to be able to use a single client with multiple drivers that, to me,
>>> is asking for a new capability bit (dma_transaction_type) rather than
>>> an id.  Do you have an example of the client and the DMACs that would
>>> first take advantage of such a cross DMAC compatibility?
>>>
>> Apparently I fail to explain my well. let me ask you this....
>>
>> Some DMAC drivers initialize chan_id before calling dma_async_device_register().
>> And dma_async_device_register() overwrites the chan_id _always_.
>>
>> Clearly only _one_ of them should be setting chan_id. Which was meant to be?
>
> Correct, it is meant that chan_id is only a sysfs property.  Any
> driver usage that is assuming chan_id is anything more than a
> guaranteed unique number within a given dma_device's list of channels
> is probably inferring too much.

So you mean dmac/client drivers are wrong if they make use of chan_id.
They shouldn't count upon it's value - which is set by DMA API for a completely
independent purpose, i.e, creating contiguous sysfs entries.

Since "chan_id is only a sysfs property" and the fact that it is used
only _once_
by the DMA API

In drivers/dma/dmaengine.c

      chan->chan_id = chancnt++;
      dev_set_name(&chan->dev->device, "dma%dchan%d",
                             device->dev_id, chan->chan_id);


Can't we do away with chan_id altogether ? by having

      dev_set_name(&chan->dev->device, "dma%dchan%d",
                             device->dev_id, chancnt++);

I mean why make every instance of dma_chan bigger by 4bytes ?

So why shouldn't we remove chan_id completely from the DMA API ?

Thanks,
Jassi


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