[U-Boot] serial atag tag in devicetree ?

[Paul Kocialkowski]

Le jeudi 26 mars 2015 à 09:53 +0100, Hans de Goede a écrit :
> Hi,
> 
> On 25-03-15 23:35, Paul Kocialkowski wrote:
> > Le mardi 24 mars 2015 à 09:01 +0100, Hans de Goede a écrit :
> >> Hi,
> >>
> >> On 24-03-15 00:12, Rob Herring wrote:
> >>> On Mon, Mar 23, 2015 at 6:30 AM, Hans de Goede <hdegoede at redhat.com> wrote:
> >>>> Hi,
> >>>>
> >>>>
> >>>> On 22-03-15 22:01, Rob Herring wrote:
> >>
> >> <snip>
> >>
> >>>>> There is already "serial-number" (a string) which exists for
> >>>>> OpenFirmware. Also, "copyright" corresponds to vendor/manufacturer
> >>>>> string. Both of these are supported by lshw already.
> >>>>
> >>>>
> >>>> Ok, so if I understand you correctly then you're saying that we
> >>>> should set a "serial-number" string property at the dt root level
> >>>> and that this may contain pretty much anything, e.g. in the
> >>>> sunxi case the full 128 bit SID in hex.
> >>>
> >>> Right.
> >>>
> >>>> Is the use of the "serial-number" string property already documented
> >>>> somewhere? If not I'll submit a kernel patch to document it.
> >>>
> >>> Not that I'm aware of. It is something that predates our documentation
> >>> requirements. It could be in OpenFirmware specs. Documenting it in the
> >>> DT bindings does not hurt.
> >>
> >> Ok.
> >>
> >>>> And for older kernels we should not set any serial atag (u-boot
> >>>> always sets it, so this leaves it at 0) and old kernel users are
> >>>> out of luck wrt getting to the serial ?
> >>>
> >>> If there is sufficient reason to support this on old kernels you could.
> >>
> >> One problem with supporting this for older kernels is that if a non 0
> >> serial gets shown in /proc/cpuinfo with older atag booted kernels, we
> >> should really show the same number in /proc/cpuinfo which means adding
> >> code to the kernel to get the devicetree "serial-number" string property
> >> and somehow put that into the 64 bits which we have in /proc/cpuinfo,
> >> but given that the "serial-number" string could be hex or decimal or
> >> what ever and > 64 bits that will likely require a platform specific
> >> solution. All doable, but the question then becomes is this worth the
> >> effort ?
> >
> > After investigating a bit more, I found out that the USB serial number
> > is expected to be a string of 32 bytes, so a 128 bit numeric serial
> > doesn't fit (it takes 32 bytes for the hex representation of 128 bits,
> > so there is no room left for the terminating null byte), hence it makes
> > sense to keep a 64 bit limitation for the serial number, if users are
> > going to rely on it as USB serial string. Moreover, it seems that
> > Android devices are mostly used 64 bit numbers for serial numbers/
> >
> > I was initially going to suggest that we set it in stone that serial
> > must be 64 numeric bits (as it was in the ATAGs days) and that
> > bootloaders would pass it that way to the kernel through device tree
> > (with two 32 bits numeric integers), but Hans talked me out of it.
> > I just want to expose the situation (especially the USB and Android
> > thing) here to double-check that everyone still is convinced that a
> > string approach in device tree is best (which is fine with me).
> 
> There are already existing users of the serial-number property in devicetree,
> and these already use a free-format string, so AFAICT we have no choice
> but to do the same as the existing users.
> 
> But Rob is the expert here, so lets see what Rob has to say.
> 
> > This way, users that still want to use the serial passed through device
> > tree as a USB serial number will have to use a string of 32 bits,
> > including the null terminating byte (which is what I'll suggest for
> > sunxi by using only 64 bits for the serial number).
> >
> > Also, I suggest that we show that serial-number string as-is in cpuinfo
> > as well
> 
> We cannot do that because we must guarantee that the serial shown
> in cpu info is a 64 bits / 16 hex values (0 padded) number, anything
> else would break the kernel <-> userspace API and potentially break
> userspace apps. So we must do the devicetree -> serialnumber low/high
> -> /proc/cpinfo version to guarantee that this format does not change.
> 
> And as discussed before if you want a non 0 serial in cpuinfo then
> the devicetree -> serialnumber low/high should be done in sunxi
> specific kernel code, as on sunxi we will know that the string in
> devicetree will be a hex value, but we've no such guarantee for
> other platforms, so we cannot simply have a generic function to
> populate erialnumber low/high from the devicetree serial-number
> string.
> 
>  > and instead make a string out of the serial ATAG in the kernel
> > prior to showing it in cpuinfo (as opposed to translating the string
> > coming from device tree to a numeric value that cpuinfo will end up
> > showing as a string at the end of the day). Thus, the serial number
> > coming from device tree will still be shown in cpuinfo as well and no
> > ABI gets broken.
> 
> You're forgetting the userspace <-> kernel ABI here, the serial line
> in /proc/cpuinfo is not a free form string it is a 64 bit int shown
> as 0 padded hex, and we cannot change that as changing that would be
> an ABI break.

IMHO this really is all about interpretation. If you consider that the
serial is already a *string* and not a hex-representation of a number
(which it is when using ATAGs, but has no reason to be in general), then
my suggestion will introduce no ABI break.

Generally speaking, I found no documentation that indicates that the
serial has to be in that format. It just happens to be the case when
using ATAGs.

Also, I found an email from Rob suggesting he would be fine with wiring
the dts serial-number string to cpuinfo:
http://lkml.iu.edu/hypermail/linux/kernel/1412.0/02975.html

I think it's the most flexible solution and we can think of it as an
extension of the current scheme: the serial string will no longer be
limited to a hex representation of a number but can become any string.

Now I would appreciate it if Rob could weigh-in and state whether he
changed his mind on this or not.

> > If you're all okay with this, I'll be sending patches to both U-Boot and
> > Linux to start documenting/implementing this.
> 
> Thanks for your work on this, lets first hash out to few remaining unclear
> details and then I'm looking forward to your patch set for this.
> 
> Regards,
> 
> Hans

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