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

[Vinod <vkoul@kernel.org>]

On 17-07-18, 11:40, Rob Herring wrote:
> On Tue, Jul 17, 2018 at 9:34 AM Vinod <vkoul@kernel.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.

Sounds good to me, thanks for the guide.

-- 
~Vinod