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

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Message-ID: <f08b1078-fc8c-4834-984c-813e01291033@kernel.org>
Date: Mon, 27 Oct 2025 20:24:41 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Ryan Chen <ryan_chen@...eedtech.com>, bmc-sw@...eedtech.com,
 benh@...nel.crashing.org, joel@....id.au, andi.shyti@...nel.org,
 jk@...econstruct.com.au, robh@...nel.org, krzk+dt@...nel.org,
 conor+dt@...nel.org, andrew@...econstruct.com.au, p.zabel@...gutronix.de,
 andriy.shevchenko@...ux.intel.com, naresh.solanki@...ements.com,
 linux-i2c@...r.kernel.org, openbmc@...ts.ozlabs.org,
 devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-aspeed@...ts.ozlabs.org, linux-kernel@...r.kernel.org
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

Best regards,
Krzysztof