Subject: Re: [PATCH 1/5] pci: introduce pci_get_dsn

From: Bjorn Helgaas <[email-masked]>
Date: Mon, 2 Mar 2020 16:25:10 -0600

PCI: Introduce pci_get_dsn()

I learned this from "git log --oneline drivers/pci/pci.c".  It looks
like the other patches could benefit from this as well.

On Thu, Feb 27, 2020 at 02:36:31PM -0800, Jacob Keller wrote:
> Several device drivers read their Device Serial Number from the PCIe
> extended config space.
> 
> Introduce a new helper function, pci_get_dsn, which will read the
> eight bytes of the DSN into the provided buffer.

"pci_get_dsn()"

> Signed-off-by: Jacob Keller <[email-masked]>
> Cc: Bjorn Helgaas <[email-masked]>
> Cc: Jeff Kirsher <[email-masked]>
> Cc: [email-masked]
> Cc: Michael Chan <[email-masked]>
> ---
>  drivers/pci/pci.c   | 33 +++++++++++++++++++++++++++++++++
>  include/linux/pci.h |  5 +++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d828ca835a98..12d8101724d7 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -33,6 +33,7 @@
>  #include <linux/pci-ats.h>
>  #include <asm/setup.h>
>  #include <asm/dma.h>
> +#include <asm/unaligned.h>
>  #include <linux/aer.h>
>  #include "pci.h"
>  
> @@ -557,6 +558,38 @@ int pci_find_ext_capability(struct pci_dev *dev, int cap)
>  }
>  EXPORT_SYMBOL_GPL(pci_find_ext_capability);
>  
> +/**
> + * pci_get_dsn - Read the 8-byte Device Serial Number
> + * @dev: PCI device to query
> + * @dsn: storage for the DSN. Must be at least 8 bytes
> + *
> + * Looks up the PCI_EXT_CAP_ID_DSN and reads the 8 bytes into the dsn storage.
> + * Returns -EOPNOTSUPP if the device does not have the capability.
> + */
> +int pci_get_dsn(struct pci_dev *dev, u8 dsn[])
> +{
> +	u32 dword;
> +	int pos;
> +
> +
> +	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DSN);
> +	if (!pos)
> +		return -EOPNOTSUPP;
> +
> +	/*
> +	 * The Device Serial Number is two dwords offset 4 bytes from the
> +	 * capability position.
> +	 */
> +	pos += 4;
> +	pci_read_config_dword(dev, pos, &dword);
> +	put_unaligned_le32(dword, &dsn[0]);
> +	pci_read_config_dword(dev, pos + 4, &dword);
> +	put_unaligned_le32(dword, &dsn[4]);

Since the serial number is a 64-bit value, can we just return a u64
and let the caller worry about any alignment and byte-order issues?

This would be the only use of asm/unaligned.h in driver/pci, and I
don't think DSN should be that special.

I think it's OK if we return 0 if the device doesn't have a DSN
capability.  A DSN that actually contains a zero serial number would
be dubious at best.

> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(pci_get_dsn);
> +
>  static int __pci_find_next_ht_cap(struct pci_dev *dev, int pos, int ht_cap)
>  {
>  	int rc, ttl = PCI_FIND_CAP_TTL;
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 3840a541a9de..883562323df3 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1045,6 +1045,8 @@ int pci_find_ht_capability(struct pci_dev *dev, int ht_cap);
>  int pci_find_next_ht_capability(struct pci_dev *dev, int pos, int ht_cap);
>  struct pci_bus *pci_find_next_bus(const struct pci_bus *from);
>  
> +int pci_get_dsn(struct pci_dev *dev, u8 dsn[]);
> +
>  struct pci_dev *pci_get_device(unsigned int vendor, unsigned int device,
>  			       struct pci_dev *from);
>  struct pci_dev *pci_get_subsys(unsigned int vendor, unsigned int device,
> @@ -1699,6 +1701,9 @@ static inline int pci_find_next_capability(struct pci_dev *dev, u8 post,
>  static inline int pci_find_ext_capability(struct pci_dev *dev, int cap)
>  { return 0; }
>  
> +static inline int pci_get_dsn(struct pci_dev *dev, u8 dsn[])
> +{ return -EOPNOTSUPP; }
> +
>  /* Power management related routines */
>  static inline int pci_save_state(struct pci_dev *dev) { return 0; }
>  static inline void pci_restore_state(struct pci_dev *dev) { }
> -- 
> 2.25.0.368.g28a2d05eebfb
>