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

[Jacob Keller <jacob.e.keller@intel.com>]

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?

Thanks,
Jake