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

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Message-ID: <20250718125935-75fa0343-af78-42be-bc3f-e8f806a4aee5@linutronix.de>
Date: Fri, 18 Jul 2025 13:02:00 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Matthew Wood <thepacketgeek@...il.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 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");

> 
> 
> > +}
> > +static DEVICE_ATTR_ADMIN_RO(serial_number);
> 
>