Re: [PATCH v3 1/1] device-types/blk/description.tex: Allow longer device IDs to be returned

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Sep 08, 2026 at 11:14:46AM -0400, Stefan Hajnoczi wrote:
> On Tue, Sep 08, 2026 at 10:40:50AM +0100, Richard W.M. Jones wrote:
> > +Although 247 byte device ID strings are allowed, there may be
> > +interoperability problems if strings longer than 128 bytes are used.
> 
> Can you be more specific? Is the concern that the guest software stack
> above of the driver may not be prepared for serial strings longer than
> 128 bytes?

I'll add more detail in the next version, but in brief the problems
are twofold:

(1) Windows supports up to 128 "characters" (they're not precise but
they probably mean Unicode codepoints from the Basic Multilingual Plane):

https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/storport/ns-storport-stor_serial_number

(2) When udev tries to encode a very long serial into a
/dev/disk/by-id path you end up uncomfortably close to NAME_MAX (255).

> > +It is also advisable to use only 7 bit ASCII characters.
> 
> Versus "The device ID string is an ASCII string which can be up to 247
> bytes long" earlier in this patch. Is ASCII a "SHOULD" or a "MUST"?

It's "SHOULD".  I can't see any good coming from trying to use
anything except letters, numbers and dashes in a serial, but both
Linux and Windows (see above) could in theory handle Unicode.

Rich.

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