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

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Message-ID: <CAJe_ZhdHcsBNT6iX7TJCqoGZMrPwE2BOt06e8M8PHzuDe3Z70w@mail.gmail.com>
Date:	Tue, 18 Oct 2011 14:00:45 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	Russell King <rmk@....linux.org.uk>
Cc:	"Bounine, Alexandre" <Alexandre.Bounine@....com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	Vinod Koul <vinod.koul@...el.com>,
	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 18 October 2011 13:12, Russell King <rmk@....linux.org.uk> wrote:
> On Tue, Oct 18, 2011 at 11:15:29AM +0530, Jassi Brar wrote:
>> On 18 October 2011 02:37, Bounine, Alexandre <Alexandre.Bounine@....com> wrote:
>> > With item #1 above being a separate topic, I may have a problem with #2
>> > as well: dma_addr_t is sized for the local platform and not guaranteed
>> > to be a 64-bit value (which may be required by a target).
>> > Agree with #3 (if #1 and #2 work).
>> >
>> Perhaps simply change dma_addr_t to u64 in dmaengine.h alone ?
>
> That's just an idiotic suggestion - there's no other way to put that.
> Let's have some sanity here.
>
Yeah, I am not proud of the workaround, so I only probed the option.
I think I need to explain myself.

The case here is that even a 32-bit RapidIO host could ask transfer against
64-bit address space on a remote device. And vice versa 64->32.

> dma_addr_t is the size of a DMA address for the CPU architecture being
> built.  This has no relationship to what any particular DMA engine uses.
>
Yes, so far the dmaengine ever only needed to transfer within platform's
address-space. So the assumption that src and dst addresses could
be contained within dma_addr_t, worked.
If the damengine is to get rid of that assumption/constraint, the memcpy,
slave_sg etc need to accept addresses specified in bigger of the host and
remote address space, and u64 is the safe option.
Ultimately dma_addr_t is either u32 or u64.

If you still think that's unacceptable, please do show us the optimal
path forward.
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