[RFC/PATCH 0/7] Powerpc MSI Implementation

[Segher Boessenkool]

> Looking closer most of them are 5 bits, the high 5 bits, and happen to
> sit next to reserved fields (which must be zero), so reading the byte
> should work in practice. But for a few of them you'll get cruft in the
> low bits.
>
> I don't know what they were thinking when they decided to have 3 and 5
> bit capability fields, and then specify some of them as being a byte
> wide as well. Perhaps the spec committee had a big night out ;)
>
> I was going to write a generic version of pci_find_ht_capability() (as
> suggested by Segher), so along with that I'll clean up the #defines to
> just be the 3 or 5 bit capability codes, and then have a shift for
> getting the capability out of the byte.
>
> Users will still need to know if they're looking for a 3 or 5 bit
> capability, but we can encapsulate that in pci_find_ht_capability()  
> and
> hopefully most people won't have to see the difference.

Keep the defines the full 8 bits and put all the knowledge
about which bits are relevant into pci_find_ht_capability()?
If you do shifting you can get clashes (say, 0b00110 vs. 0b110),
and this would make for simpler code, too.


Segher