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Date:   Mon, 18 Feb 2019 13:23:00 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Baolin Wang <baolin.wang@...aro.org>
Cc:     Vinod Koul <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Olof Johansson <olof@...om.net>,
        Orson Zhai <orsonzhai@...il.com>,
        Lyra Zhang <zhang.lyra@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        DTML <devicetree@...r.kernel.org>, arm-soc <arm@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dmaengine@...r.kernel.org, eric.long@...soc.com,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 1/3] dt-bindings: dmaengine: Add one new cell to present
 hardware slave id

On Mon, Feb 18, 2019 at 11:52 AM Baolin Wang <baolin.wang@...aro.org> wrote:
> On Mon, 18 Feb 2019 at 18:31, Arnd Bergmann <arnd@...db.de> wrote:
> >
> > On Tue, Feb 12, 2019 at 9:25 AM Baolin Wang <baolin.wang@...aro.org> wrote:
> > > On Fri, 1 Feb 2019 at 19:53, Baolin Wang <baolin.wang@...aro.org> wrote:
> > > > On Thu, 31 Jan 2019 at 00:52, Arnd Bergmann <arnd@...db.de> wrote:
> > > > > On Tue, Jan 22, 2019 at 2:21 PM Baolin Wang <baolin.wang@...aro.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@...db.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.

      Arnd

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