linux-kernel - Re: [PATCH 2/8 v2] Documentation: bindings: k3dma: Add binding for dma-avail-chan

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Date:   Sat, 5 Jan 2019 10:23:20 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>, Vinod Koul <vkoul@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Tanglei Han <hantanglei@...wei.com>,
        Zhuangluan Su <suzhuangluan@...ilicon.com>,
        Ryan Grachek <ryan@...ted.us>, dmaengine@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH 2/8 v2] Documentation: bindings: k3dma: Add binding for
 dma-avail-chan

On Fri, Jan 04, 2019 at 08:39:34PM -0800, John Stultz wrote:
> On Fri, Jan 4, 2019 at 8:00 PM Manivannan Sadhasivam
> <manivannan.sadhasivam@...aro.org> wrote:
> >
> > Hi John,
> >
> > On Fri, Jan 04, 2019 at 12:56:22PM -0800, John Stultz wrote:
> > > Some dma channels can be reserved for secure mode or other
> > > hardware on the SoC, so provide a binding for a bitmask
> > > listing the available channels for the kernel to use.
> > >
> > > Cc: Vinod Koul <vkoul@...nel.org>
> > > Cc: Rob Herring <robh+dt@...nel.org>
> > > Cc: Mark Rutland <mark.rutland@....com>
> > > Cc: Tanglei Han <hantanglei@...wei.com>
> > > Cc: Zhuangluan Su <suzhuangluan@...ilicon.com>
> > > Cc: Ryan Grachek <ryan@...ted.us>
> > > Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > Cc: dmaengine@...r.kernel.org
> > > Cc: devicetree@...r.kernel.org
> > > Signed-off-by: John Stultz <john.stultz@...aro.org>
> > > ---
> > >  Documentation/devicetree/bindings/dma/k3dma.txt | 3 +++
> > >  1 file changed, 3 insertions(+)
> > >
> > > diff --git a/Documentation/devicetree/bindings/dma/k3dma.txt b/Documentation/devicetree/bindings/dma/k3dma.txt
> > > index 10a2f15..1c466c1 100644
> > > --- a/Documentation/devicetree/bindings/dma/k3dma.txt
> > > +++ b/Documentation/devicetree/bindings/dma/k3dma.txt
> > > @@ -14,6 +14,9 @@ Required properties:
> > >               have specific request line
> > >  - clocks: clock required
> > >
> > > +Optional properties:
> > > +- dma-avail-chan: Bitmask of available physical channels
> > > +
> >
> > This property looks too generic. Since this is specific to HiSi SoCs,
> > this could be "hisi-dma-avail-chan"?
> 
> I'm fine to change it, but I'm not sure I fully understand the
> rational. Can you help me understand?
> Are device node-binding names supposed to have global scope? I assumed
> the node property names are basically scoped to the entry?

IIUC properties documented in subsystem binding (dma.txt in this case)
will have global scope. Those which are not documented in this binding
are specific to vendor IPs and should be prefixed with the vendor
prefix (hisi in this case).

> Further, having some dma channels be reserved doesn't seem to be too
> unique a concept, so I'm not sure what we gain long term by prefixing
> it?
> 

Right, but this brings up the point of having this functionality in
generic DMA engine so that the DMA controller drivers need not handle.
So either we should move this available channel check to DMA Engine
and document the property in dma.txt so that other IPs can also use it
or keep the functionality in K3 driver and use HiSi prefix for the
property.

But I'd like to hear Vinod/Rob's opinion on this!

Thanks,
Mani

> thanks
> -john