linux-kernel - RE: [PATCH v21 3/4] i2c: ast2600: Add controller driver for new register layout

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Message-ID:
 <TY2PPF5CB9A1BE64FEBCCCDC7631B355135F2C3A@TY2PPF5CB9A1BE6.apcprd06.prod.outlook.com>
Date: Fri, 7 Nov 2025 06:26:39 +0000
From: Ryan Chen <ryan_chen@...eedtech.com>
To: Krzysztof Kozlowski <krzk@...nel.org>, BMC-SW <BMC-SW@...eedtech.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>, "joel@....id.au"
	<joel@....id.au>, "andi.shyti@...nel.org" <andi.shyti@...nel.org>,
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	<robh@...nel.org>, "krzk+dt@...nel.org" <krzk+dt@...nel.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>, "andrew@...econstruct.com.au"
	<andrew@...econstruct.com.au>, "p.zabel@...gutronix.de"
	<p.zabel@...gutronix.de>, "andriy.shevchenko@...ux.intel.com"
	<andriy.shevchenko@...ux.intel.com>, "naresh.solanki@...ements.com"
	<naresh.solanki@...ements.com>, "linux-i2c@...r.kernel.org"
	<linux-i2c@...r.kernel.org>, "openbmc@...ts.ozlabs.org"
	<openbmc@...ts.ozlabs.org>, "devicetree@...r.kernel.org"
	<devicetree@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-aspeed@...ts.ozlabs.org"
	<linux-aspeed@...ts.ozlabs.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v21 3/4] i2c: ast2600: Add controller driver for new
 register layout

> Subject: Re: [PATCH v21 3/4] i2c: ast2600: Add controller driver for new
> register layout
> 
> On 27/10/2025 07:12, Ryan Chen wrote:
> > Add i2c-ast2600 new register mode driver to support AST2600 i2c new
> > register mode. This i2c-ast2600 new driver and the legacy i2c-aspeed
> > driver both match the same compatible string "aspeed,ast2600-i2c-bus"
> > because they target the same I2C controller IP on AST2600. However,
> > AST2600 SoCs may configure
> 
> Where did you document ABI compatibility of new driver with old DTS, which
> do not have required properties and uses old reg?
> 
> I don't see that. I actually clearly see:
> 
> > +	i2c_bus = devm_kzalloc(dev, sizeof(*i2c_bus), GFP_KERNEL);
> > +	if (!i2c_bus)
> > +		return -ENOMEM;
> > +
> > +	i2c_bus->reg_base = devm_platform_ioremap_resource(pdev, 0);
> > +	if (IS_ERR(i2c_bus->reg_base))
> > +		return PTR_ERR(i2c_bus->reg_base);
> > +
> > +	rst = devm_reset_control_get_shared_deasserted(dev, NULL);
> > +	if (IS_ERR(rst))
> > +		return dev_err_probe(dev, PTR_ERR(rst), "Missing reset ctrl\n");
> > +
> > +	i2c_bus->global_regs =
> > +		syscon_regmap_lookup_by_phandle(dev_of_node(dev),
> "aspeed,global-regs");
> > +	if (IS_ERR(i2c_bus->global_regs))
> > +		return PTR_ERR(i2c_bus->global_regs);
> 
> 
> ABI break. No explanation of that ABI break, not even mentioning it.
> 
> There is no reason to break the ABI. Your new driver *must* support both
> interfaces and both DTB. You *cannot* (please listen carefully, because in the
> past we wasted a lot of time discussing simple statements) rely on legacy
> driver binding to old DTB, because it is the same compatible.
> 
> You need to rewrite all this to handle both DTBs in backwards compatible way.
> 
> NAK
I think I understand your point about ABI break.
I will try to merge i2c-ast260.c with legacy i2c-aspeed.c
Add new file i2c-aspeed-core.c to do legacy probe and i2c-ast2600 probe.

	if (of_device_is_compatible(dev_of_node(dev), "aspeed,ast2600-i2c-bus") &&
	    of_parse_phandle(dev_of_node(dev), "aspeed,global-regs", 0)) {
		ret = ast2600_i2c_probe(pdev);
	} else {
		ret = aspeed_i2c_probe(pdev);
	}

That would not have ABI break.

> 
> Best regards,
> Krzysztof