Re: [PATCH 1/3] dt-bindings: dmaengine: Add one new cell to present hardware slave id

[Baolin Wang]

On Wed, 20 Feb 2019 at 17:08, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Feb 20, 2019 at 4:13 AM Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> > On Tue, 19 Feb 2019 at 20:20, Vinod Koul <vkoul@xxxxxxxxxx> wrote:
> > > On 19-02-19, 17:49, Baolin Wang wrote:
> > > > On Tue, 19 Feb 2019 at 17:30, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
> > > > > On Tue, Feb 19, 2019 at 4:15 AM Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> > > > > > On Mon, 18 Feb 2019 at 20:23, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > > > > On Mon, Feb 18, 2019 at 11:52 AM Baolin Wang <baolin.wang@xxxxxxxxxx> wrote:
> > > > > > > > On Mon, 18 Feb 2019 at 18:31, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > > > > > > I did understand the need for a slave-id, I was instead wondering about
> > > > > > > the channel-id number. On many SoCs, all channels are equal, and you
> > > > > > > just have to pick one of those with the right capabilities for a particular
> > > > > > > slave.
> > > > > >
> > > > > > Yes, all channels are equal. We just set a unique slave id for the
> > > > > > channel for a particular slave. For example, the SPI slave can use
> > > > > > channel 10 for tx transfer by setting slave id 11, or it also can use
> > > > > > channel 9 for tx transfer by setting same slave id 11.
> > > > >
> > > > > So the channel selection is software policy, not hardware description, and
> > > > > thus doesn't belong in DT?
> > > > >
> > > > > Can't the DMA engine driver allocate channels dynamically, removing the
> > > > > need to specify this in DT?
> > > >
> > > > In theory we can do as you suggested. But we still want to
> > > > manage/assign the DMA channel resources manually for one SoC, we can
> > > > make sure some important DMA slaves (such as audio)  can request a DMA
> > > > channel at runtime firstly, another benefit is that it is easy to
> > > > debug since we can easily know which channel is assigned for this
> > > > slave.
> > >
> > > Are  you suggesting that you have more users than channels available?
> >
> > Until now we have not met this issue, but we can not make sure if this
> > can happen in future. Moreover DMA channel resources are belonging to
> > the DMA engine's hardware resources, I think it should be described in
> > DT like many other drivers did.
>
> As far as I can tell, most platforms do not describe the assignment
> of resources in DT for dma engines, the numbers in there are meant to
> describe whatever is fixed, and most platforms should do it that way.
>
> The naming is sometimes a bit confusing, as a dma request line
> assignment can be called a slave-id or a channel-id depending whose
> documentation you read. The drivers/dma/virt-dma.c implementation
> is used in some cases to represent request numbers as virtual
> channels, so a dmaengine driver can allow one dma_request_chan()
> per slave, and then assign the hardware channels dynamically
> while doing a transfer, rather than having a fixed channel assignment
> when we first ask for a channel.

Okay, sounds reasonable to me, and I think no issues will happen if we
assign channels dynamically after some slave usages' investigation.

I will remove channel id from DT in next version. Thanks for all your help.

-- 
Baolin Wang
Best Regards