linux-kernel - Re: [PATCH v5 1/1] PCI/sysfs: Expose PCIe device serial number

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Message-ID: <CADvopvapHnuxztum4fPsZU5h3=977Y=h6xOVhyWfKU8tQ0wxeQ@mail.gmail.com>
Date: Fri, 18 Jul 2025 09:27:10 -0700
From: Matthew Wood <thepacketgeek@...il.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Mario Limonciello <superm1@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-pci@...r.kernel.org
Subject: Re: [PATCH v5 1/1] PCI/sysfs: Expose PCIe device serial number

On Fri, Jul 18, 2025 at 4:02 AM Thomas Weißschuh
<thomas.weissschuh@...utronix.de> wrote:
>
> On Fri, Jul 18, 2025 at 11:36:11AM +0100, Jonathan Cameron wrote:
> > On Thu, 17 Jul 2025 09:50:54 -0700
> > Matthew Wood <thepacketgeek@...il.com> wrote:
>
> (...)
>
> > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > > index 268c69daa4d5..bc0e0add15d1 100644
> > > --- a/drivers/pci/pci-sysfs.c
> > > +++ b/drivers/pci/pci-sysfs.c
> > > @@ -239,6 +239,22 @@ static ssize_t current_link_width_show(struct device *dev,
> > >  }
> > >  static DEVICE_ATTR_RO(current_link_width);
> > >
> > > +static ssize_t serial_number_show(struct device *dev,
> > > +                                  struct device_attribute *attr, char *buf)
> > > +{
> > > +   struct pci_dev *pci_dev = to_pci_dev(dev);
> > > +   u64 dsn;
> > > +
> > > +   dsn = pci_get_dsn(pci_dev);
> > > +   if (!dsn)
> > > +           return -EIO;
> > > +
> > > +   return sysfs_emit(buf, "%02llx-%02llx-%02llx-%02llx-%02llx-%02llx-%02llx-%02llx\n",
> > > +           dsn >> 56, (dsn >> 48) & 0xff, (dsn >> 40) & 0xff, (dsn >> 32) & 0xff,
> > > +           (dsn >> 24) & 0xff, (dsn >> 16) & 0xff, (dsn >> 8) & 0xff, dsn & 0xff);
> >
> > I wonder if doing the following i too esoteric. Eyeballing those shifts is painful.
> >
> >       u8 bytewise[8]; /* naming hard... */
> >
> >       put_unaligned_u64(dsn, bytewise);
> >
> >       return sysfs_emit(buf, "%02x-%02x-%02x-%02x-%02x-%02x-%02x-%02x\n",
> >               bytewise[0], bytewise[1], bytewise[2], bytewise[3],
> >               bytewise[4], bytewise[5], bytewise[6], bytewise[7]);
>
> This looks endianess-unsafe.
>
> Maybe just do what some drivers are doing:
>
>         u8 bytes[8];
>
>         put_unaligned_be64(dsn, bytes);
>
>         return sysfs_emit(buf, "%8phD");

Thank you both for your continued review! That reads much nicer, I
should've known to look at how others were doing this formatting. I'll
have a new patch later today.