Re: SoC-specific device tree aliases?

Marc Kleine-Budde <[email protected]> Wed, 3 Dec 2025 12:20:52 +0100

Newsgroups	org.kernel.vger.devicetree-spec,org.kernel.vger.linux-devicetree
Message-ID	<[email protected]>

On 03.12.2025 11:25:11, Krzysztof Kozlowski wrote:
> On 03/12/2025 11:16, Ahmad Fatoum wrote:
> > Hello Krzysztof,
> >
> > On 11/17/25 5:29 PM, Krzysztof Kozlowski wrote:
> >> On 17/11/2025 17:06, Rob Herring wrote:
> >>>> So you want it to be an ABI for barebox, sure, just make it a binding.
> >>>
> >>> What do you have in mind? Other than standard names for the aliases,
> >>> what can we check here? That a specific alias points to a specific
> >>> path? That would be a bit too much IMO. That would be equivalent to
> >>> specifying possible values in 'reg' for all devices.
> >>
> >> Binding with pattern or list of needed alias names, referenced by given
> >> soc-platform top-level schema.
> >>
> >> One of the points is to make it explicit and obvious (e.g. to Arnd or to
> >> me if I forget, because I follow the same logic of aliases per board)
> >> that these aliases are used outside of kernel.
> >>
> >> Just because ufs/mmc/spi can be used that way, does not mean we should
> >> accept any possible alias into soc.dtsi.
> >
> > I can't see how this could work. A number of boards renumber MMC devices
> > in a different manner than the SoC reference manual:
> >
> > - Changing the alias numbering is an ABI break, because Linux derives
> > its /dev/mmcblkX numbering from it
>
> First, why the alias would change? Isn't the board following the SoC
> numbering in 99.9% cases?
>
> Second, I don't think it is an ABI. We had it ~5 or ~8 years ago where
> the mmcblkX was changing based on probe ordering. Many people setups got
> broken, many people complained and the consensus reply was: please start
> finally using UUID/PARTUIID/LABEL for rootfs. Eventually we got back to
> mmcblkX stability but rule is there - if your cmdline has /dev/mmcblkX,
> then it is your problem.

In order to use UUID/PARTUIID/LABEL in the kernel command line, the
firmware/bootloader/... has to map from the register value to the
instance to read the UUID/PARTUIID/LABEL from the media.

Marc

-- 
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