netdev - Re: [PATCH 1/5] pci: introduce pci_get

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Date:   Mon, 2 Mar 2020 17:39:58 -0600
From:   Bjorn Helgaas <bjorn.helgaas@...il.com>
To:     Jacob Keller <jacob.e.keller@...el.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        netdev@...r.kernel.org, Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        QLogic-Storage-Upstream@...ium.com,
        Michael Chan <michael.chan@...adcom.com>
Subject: Re: [PATCH 1/5] pci: introduce pci_get_dsn

On Mon, Mar 2, 2020 at 5:24 PM Jacob Keller <jacob.e.keller@...el.com> wrote:
>
> On 3/2/2020 3:20 PM, Bjorn Helgaas wrote:
> > On Mon, Mar 02, 2020 at 02:33:12PM -0800, Jacob Keller wrote:
> >> On 3/2/2020 2:25 PM, Bjorn Helgaas wrote:
> >
> >>>> +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 suppose that's fair, but it ends up leaving most callers having to fix
> >> this immediately after calling this function.
> >
> > PCIe doesn't impose any structure on the value; it just says the first
> > dword is the lower DW and the second is the upper DW.  As long as we
> > put that together correctly into a u64, I think further interpretation
> > is caller-specific.
>
> Makes sense. So basically, convert pci_get_dsn to a simply return a u64
> instead of copying to an array, and then make callers assume that a
> value of 0 is invalid?

Yep, that's what I would do.

You might have to re-jigger the snprintfs so they still pull out the
same bytes they did before.