linux-kernel - Re: [PATCH 05/14] dmaengine: dma-jz4780: Add support for the JZ4740 SoC

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Date:   Tue, 17 Jul 2018 11:40:20 -0600
From:   Rob Herring <robh@...nel.org>
To:     Vinod <vkoul@...nel.org>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        Mark Rutland <mark.rutland@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>,
        Mathieu Malaterre <malat@...ian.org>,
        Daniel Silsby <dansilsby@...il.com>,
        "open list:DMA GENERIC OFFLOAD ENGINE SUBSYSTEM" 
        <dmaengine@...r.kernel.org>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-MIPS <linux-mips@...ux-mips.org>
Subject: Re: [PATCH 05/14] dmaengine: dma-jz4780: Add support for the JZ4740 SoC

On Tue, Jul 17, 2018 at 9:34 AM Vinod <vkoul@...nel.org> wrote:
>
> On 16-07-18, 15:33, Rob Herring wrote:
> > On Mon, Jul 09, 2018 at 10:42:26PM +0530, Vinod wrote:
> > > On 03-07-18, 14:32, Paul Cercueil wrote:
> > >
> > > >  enum jz_version {
> > > > + ID_JZ4740,
> > > >   ID_JZ4770,
> > > >   ID_JZ4780,
> > > >  };
> > > > @@ -247,6 +248,7 @@ static void jz4780_dma_desc_free(struct virt_dma_desc *vdesc)
> > > >  }
> > > >
> > > >  static const unsigned int jz4780_dma_ord_max[] = {
> > > > + [ID_JZ4740] = 5,
> > > >   [ID_JZ4770] = 6,
> > > >   [ID_JZ4780] = 7,
> > > >  };
> > > > @@ -801,11 +803,13 @@ static struct dma_chan *jz4780_of_dma_xlate(struct of_phandle_args *dma_spec,
> > > >  }
> > > >
> > > >  static const unsigned int jz4780_dma_nb_channels[] = {
> > > > + [ID_JZ4740] = 6,
> > > >   [ID_JZ4770] = 6,
> > > >   [ID_JZ4780] = 32,
> > > >  };
> > >
> > > I feel these should be done away with if we describe hardware in DT
> >
> > The compatible property can imply things like this.
>
> So what is the general recommendation, let DT describe hardware
> including version delta or use compatible to code that in driver?

Compatible is the version. Looking at the above, the version or ID
isn't even stable.

> Is it documented anywhere?

Not really. It's a judgment call generally. Maybe # of DMA channels
should be a property because that is something most controllers have.
But you really have to define the property up front, not when the 2nd
version of h/w shows up with different properties.

To start defining guidelines, a couple of things come to mind:

- Define properties for parameters that vary from board to board (for one SoC).
- You can't add new required properties to existing bindings, so the
not present default must work for all existing compatibles (or you
need per compatible driver data).
- Bugs/quirks/errata should be handled by compatible, not adding a
property. Because bugs should be fixable without a dtb update and only
a kernel update.

Rob