Re: [PATCH v2 1/5] Documentation: dmaengine: pxa-dma design

From: Robert Jarzmik
Date: Tue May 12 2015 - 15:14:29 EST

Vinod Koul <vinod.koul@xxxxxxxxx> writes:

> On Fri, May 08, 2015 at 02:52:46PM +0200, Robert Jarzmik wrote:
>> Vinod Koul <vinod.koul@xxxxxxxxx> writes:
>> Euh no, I meant that a transfer which is submitted and issued on a _phy_
>> doesn't wait for a _phy_ to stop and restart, but is submitted on a "running
>> channel". The other drivers, especially mmp_pdma waited for the phy to stop
>> before relaunching a new transfer.
>> I don't have a clear idea on a better wording yet ...
> Ah okay, with that explanation it helps, can you add that to
> comments/documentation
Sure, for v3.

>> >> + This implies that the queuing doesn't wait for the previous transfer end,
>> >> + and that the descriptor chaining is not only done in the irq/tasklet code
>> >> + triggered by the end of the transfer.
>> > how is it differenat than current dmaengine semantics where you say
>> > issue_pending() is invoked when current transfer finished? Here is you have
>> > to do descriptor chaining so bit it.
>> Your sentence is a bit difficult for me to understand.
> Sorry for typo, meant:
> how is it different than current dmaengine semantics where you say
> issue_pending() is invoked when current transfer finishes? Here you are
> doing descriptor chaining, so be it.

It is not "different" from dmaengine semantics. It's an implementation choice
which is not strictly required by dmaengine, and therefore a requirement on top
of what dmaengine offers.
Dmaengine requires to submit a transfer, and gives issue_pending() to provide a
guarantee the transfer will be executed. The dmaengine drivers can choose to
either queue the transfer when the previous one's completion is notified
(interrupt), or hot-queue the transfer while the channel is running.

This constraint documents the fact that this specific dmaengine driver's
implementation chose to hot-chain transfers whenever possible.

> Ideally dmaengine driver should keep submitting txns and opportunistically
> based on HW optimize it. All this is transparent to clients, they submit and
> wait for callback.
True. Yet this is not a requirement, it's a "good design" behavior. I wonder how
many dmaengine drivers are behaving in an optimize way ...

>> >> + granularity is still descriptor based.
>> > This is not pxa specfic
>> True. Do you want me to remove the (c) from the document ?
> yes
Ok, for v3.

>> >> + f) Transfer reusability
>> >> + An issued and finished transfer should be "reusable". The choice of
>> >> + "DMA_CTRL_ACK" should be left to the client, not the dma driver.
>> > again how is this pxa specfic, if not documented we should move this to
>> > dmaengine documentation
>> Yes, I agree. I should move this to dmaengine slave documentation, in
>> Documentation/dmaengine/provider.txt (in the Misc notes section). Do you want me
>> to submit a patch to change the "Undocumented feature" into a properly
>> documented feature ?
> That would be great
On my way, for v3.


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