linux-kernel - Re: [PATCHv4] DMAEngine: Define interleaved transfer request api

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Date:	Sat, 15 Oct 2011 00:06:41 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	"Bounine, Alexandre" <Alexandre.Bounine@....com>
Cc:	"Williams, Dan J" <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	Russell King <rmk@....linux.org.uk>,
	Barry Song <21cnbao@...il.com>, linux-kernel@...r.kernel.org,
	DL-SHA-WorkGroupLinux <workgroup.linux@....com>,
	Dave Jiang <dave.jiang@...el.com>
Subject: Re: [PATCHv4] DMAEngine: Define interleaved transfer request api

On 14 October 2011 23:20, Bounine, Alexandre <Alexandre.Bounine@....com> wrote:
>> On Fri, Oct 7, 2011 at 4:27 AM, Jassi Brar
> <jaswinder.singh@...aro.org>
>> wrote:
>> > On 7 October 2011 11:15, Vinod Koul <vinod.koul@...el.com> wrote:
>> >
>> >> Thru this patch Jassi gave a very good try at merging DMA_SLAVE and
>> >> memcpy, but more we debate this, I am still not convinced about
>> merging
>> >> memcpy and DMA_SLAVE yet.
>> >>
>> > Nobody is merging memcpy and DMA_SLAVE right away.
>> > The api's primary purpose is to support interleave transfers.
>> > Possibility to merge other prepares into this is a side-effect.
>> >
>> >> I would still argue that if we split this on same lines as current
>> >> mechanism, we have clean way to convey all details for both cases.
>> >>
>> > Do you mean to have separate interleaved transfer apis for Slave
>> > and Mem->Mem ? Please clarify.
>> >
>>
>> This is a tangent, but it would be nice if this API extension also
>> covered the needs of the incoming RapidIO case which wants to specify
>> new device context information per operation (and not once at
>> configuration time, like slave case).  Would it be enough if the
>> transfer template included a (struct device *context) member at the
>> end?  Most dma users could ignore it, but RapidIO could use it to do
>> something like:
>>
>>    struct rio_dev *rdev = container_of(context, typeof(*rdev),
> device);
>>
>> That might not be enough, but I'm concerned that making the context a
>> (void *) is too flexible.  I'd rather have something like this than
>> acquiring a lock in rio_dma_prep_slave_sg() and holding it over
>> ->prep().  The alternative is to extend device_prep_slave_sg to take
>> an extra parameter, but that impacts all other slave implementations
>> with a dead parameter.
>>
>
> Having context limited to the device structure will not be enough for
> RapidIO because of 66-bit target address (dma_addr_t will not work
> here).
> Probably that range is out of practical use at this moment but it is
> defined by RIO specification and I would prefer to deal with it now
> instead of postponing it for future. Passing context using (void *) will
> solve this.
>
OK so you need a void* to contain all info. Agreed.
But doesn't the info, pointed to by this (void *), remain same for every
transfer to a particular target/remote device ?
If so, couldn't you stick this (void *) to the virtual channel's
'private' ?  'private' :D
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