linux-kernel - Re: [PATCH 02/14] dmaengine: dma-jz4780: Separate chan/ctrl registers

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Date:   Wed, 11 Jul 2018 16:27:15 -0700
From:   Paul Burton <paul.burton@...s.com>
To:     Vinod <vkoul@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     Paul Cercueil <paul@...pouillou.net>,
        Ralf Baechle <ralf@...ux-mips.org>,
        James Hogan <jhogan@...nel.org>,
        Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>,
        Mathieu Malaterre <malat@...ian.org>,
        Daniel Silsby <dansilsby@...il.com>, dmaengine@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mips@...ux-mips.org
Subject: Re: [PATCH 02/14] dmaengine: dma-jz4780: Separate chan/ctrl registers

Hi Vinod,

On Wed, Jul 11, 2018 at 05:46:55PM +0530, Vinod wrote:
> > > >  -	jzdma->base = devm_ioremap_resource(dev, res);
> > > >  -	if (IS_ERR(jzdma->base))
> > > >  -		return PTR_ERR(jzdma->base);
> > > >  +	jzdma->chn_base = devm_ioremap_resource(dev, res);
> > > >  +	if (IS_ERR(jzdma->chn_base))
> > > >  +		return PTR_ERR(jzdma->chn_base);
> > > >  +
> > > >  +	res = platform_get_resource(pdev, IORESOURCE_MEM, 1);
> > > >  +	if (!res) {
> > > >  +		dev_err(dev, "failed to get I/O memory\n");
> > > >  +		return -EINVAL;
> > > >  +	}
> > > 
> > > okay and this breaks if you happen to get probed on older DT. I think DT
> > > is treated as ABI so you need to continue support older method while
> > > finding if DT has split resources
> > 
> > See my response to PrasannaKumar. All the Ingenic-based boards do compile
> > the devicetree within the kernel, so I think it's still fine to add breaking
> > changes. I'll wait on @Rob to give his point of view on this, though.
> > 
> > (It's not something hard to change, but I'd like to know what's the policy
> > in that case. I have other DT-breaking patches to submit)
> 
> The policy is that DT is an ABI and should not break :)

I think in general that's a good policy to have for compatibility, but
if it's known for certain that the DT for all users of a driver is
always built into the kernel then I don't see why we shouldn't feel free
to change a binding. I agree with Paul that it'd be interesting to hear
the DT binding maintainers take on this.

Thanks,
    Paul