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From:   Baolin Wang <baolin.wang@linaro.org>
Date:   Tue, 19 Feb 2019 17:49:19 +0800
Message-ID: <CAMz4ku+yzwaww5o06tz9-8n99UfiVh6d5UB2L12RYMpTow6K3w@mail.gmail.com>
Subject: Re: [PATCH 1/3] dt-bindings: dmaengine: Add one new cell to present
 hardware slave id
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, Vinod Koul <vkoul@kernel.org>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Olof Johansson <olof@lixom.net>,
        Orson Zhai <orsonzhai@gmail.com>,
        Lyra Zhang <zhang.lyra@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        DTML <devicetree@vger.kernel.org>, arm-soc <arm@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dmaengine@vger.kernel.org, eric.long@unisoc.com,
        Mark Brown <broonie@kernel.org>
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Hi Geert,

On Tue, 19 Feb 2019 at 17:30, Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Baolin,
>
> On Tue, Feb 19, 2019 at 4:15 AM Baolin Wang <baolin.wang@linaro.org> wrote:
> > On Mon, 18 Feb 2019 at 20:23, Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Mon, Feb 18, 2019 at 11:52 AM Baolin Wang <baolin.wang@linaro.org> wrote:
> > > > On Mon, 18 Feb 2019 at 18:31, Arnd Bergmann <arnd@arndb.de> wrote:
> > > > >
> > > > > On Tue, Feb 12, 2019 at 9:25 AM Baolin Wang <baolin.wang@linaro.org> wrote:
> > > > > > On Fri, 1 Feb 2019 at 19:53, Baolin Wang <baolin.wang@linaro.org> wrote:
> > > > > > > On Thu, 31 Jan 2019 at 00:52, Arnd Bergmann <arnd@arndb.de> wrote:
> > > > > > > > On Tue, Jan 22, 2019 at 2:21 PM Baolin Wang <baolin.wang@linaro.org> wrote:
> > > > > > > > >
> > > > > > > > >  Client:
> > > > > > > > >  DMA clients connected to the Spreadtrum DMA controller must use the format
> > > > > > > > > -described in the dma.txt file, using a two-cell specifier for each channel.
> > > > > > > > > -The two cells in order are:
> > > > > > > > > +described in the dma.txt file, using a three-cell specifier for each channel.
> > > > > > > > > +The three cells in order are:
> > > > > > > > >  1. A phandle pointing to the DMA controller.
> > > > > > > > >  2. The channel id.
> > > > > > > > > +3. The hardware slave id which is used for clients to trigger DMA engine
> > > > > > > > > +automatically.
> > > > > > > >
> > > > > > > > I notice that this is an incompatible binding change. Is that necessary?
> > > > > > > > If the current code works, I'd suggest allowing both #dma-cells=<2>
> > > > > > > > and <3>, and then implementing both in the driver.
> > > > > > >
> > > > > > > Yes, this is necessary.
> > > > > > >
> > > > > > > Yes, current code can work, but the problem is that the DMA clients
> > > > > > > must add one property (something like "sprd,slave-id") to specify the
> > > > > > > slave id. So considering this, we want to change the dma-cells to 2,
> > > > > > > including dma channel and dma slave id, which can avoid introducing
> > > > > > > some similar properties for DMA clients.
> > > > > > >
> > > > > > > Now there are no DMA clients in mainline for Spreadtrum platform, and
> > > > > > > we want to upstream our first DMA clients: SPI controller. So no other
> > > > > > > drivers need to change when we changing dma cells. Thanks.
> > > > > >
> > > > > > Do you have any other concerns about this patch set? If not, I think
> > > > > > Vinod can apply this patch set. Thanks.
> > > > >
> > > > > Sorry for the late reply. Yes, this makes sense since there are no
> > > > > existing users then. For the DT changes going through the dmaengine
> > > > > tree
> > > > >
> > > > > Acked-by: Arnd Bergmann <arnd@arndb.de>
> > > >
> > > > Thanks for your reviewing.
> > > >
> > > > >
> > > > > One more question, to make sure we don't need to edit it again:
> > > > > Why do you need both a 'channel id' and a 'slave id' here? Is this
> > > > > a strict hardware requirement for your DMA engine? In many
> > > > > other designs, only a DMA request line number needs to
> > > > > be described, and I think this would correspond to what you
> > > > > call the 'hardware slave id' in your documentation.
> > > >
> > > > I try to explain why we need the slave id.
> > > >
> > > > For our DMA engine driver, we have software request mode and hardware
> > > > request mode. For software request mode, the DMA engine driver need
> > > > trigger DMA to start transfer manually. But for hardware request mode,
> > > > we just set one unique slave id corresponding to the slave hardware to
> > > > the DMA engine, then the slave hardware can trigger DMA automatically.
> > > > And the slave id is not always same with the channel id according to
> > > > the SoC design, so we add one cell to specify the slave id.
> > >
> > > 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.

-- 
Baolin Wang
Best Regards