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

[Arnd Bergmann]

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.

       Arnd