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

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Message-ID: <CAJe_ZheVyLk-yqyF_XO80xX3j7_7mDPnv-me+n=X_hNWycSLwg@mail.gmail.com>
Date:	Tue, 26 Jul 2011 00:47:45 +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 00:25, Williams, Dan J <dan.j.williams@...el.com> wrote:
>>> Unless you guaranteed that every id is globally unique I don't see how
>>> they are generically usable by common clients?
>>>
>> Yes. And the first step is to allow DMAC drivers to freely set chan_id value.
>> Platform could pass the list and mapping of supported 'global channels'
>> via platform_data(?) which the DMAC drivers could set in chan_id
>> And I am not sure of defining a new variable for that, because chan_id
>> is actually used only by some dmac drivers for internal purpose only -
>> which they could do by private variables.
>
> Yes, it does appear to have grown a lot of dubious usage in
> drivers/dma/ for something that is described as just a "channel ID for
> sysfs".  The intent was for drivers that need to maintain their own
> hardware specific identification to define a local id scheme (like
> iop-adma does in the iop3xx case).

You mean chan_id is meant to be used for _internal_ purposes by DMAC drivers ?

Shouldn't there instead be just something like 'void *dmac_data' for
DMAC drivers to hang their private info on?

Sorry, I couldn't find chan_id in drivers/dma/iop-adma.c
Do I misunderstand ?

>
> 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?


>> Proposal to have global cross-platform enum of channel-IDs defined by
>> client drivers, was to be my next patch.
>> Though I think, this patch is valid in it's own light.
>
> This is not the purpose of chan_id.
Let us ignore it for a while.
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