[U-Boot] serial atag tag in devicetree ?

[Hans de Goede]

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.

> 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