Re: [Xen-devel] [PATCH v2 01/12] pci: introduce a devfn field to pci_sbdf_t

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Jun 06, 2019 at 11:50:06AM +0200, Paul Durrant wrote:
> > -----Original Message-----
> > From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf 
> > Of Roger Pau Monne
> > Sent: 06 June 2019 10:02
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; 
> > Konrad Rzeszutek Wilk
> > <konrad.wilk@xxxxxxxxxx>; George Dunlap <George.Dunlap@xxxxxxxxxx>; Andrew 
> > Cooper
> > <Andrew.Cooper3@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Tim 
> > (Xen.org) <tim@xxxxxxx>; Julien
> > Grall <julien.grall@xxxxxxx>; Jan Beulich <jbeulich@xxxxxxxx>; Roger Pau 
> > Monne <roger.pau@xxxxxxxxxx>
> > Subject: [Xen-devel] [PATCH v2 01/12] pci: introduce a devfn field to 
> > pci_sbdf_t
> > 
> > This is equivalent to the current extfunc field in term of contents.
> > 
> > Switch the two current users of extfunc to use devfn instead for
> > correctness.
> > 
> > No functional change.
> > 
> > Requested-by: Jan Beulich <jbeulich@xxxxxxxx>
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Julien Grall <julien.grall@xxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Cc: Tim Deegan <tim@xxxxxxx>
> > Cc: Wei Liu <wl@xxxxxxx>
> > ---
> > Changes since v1:
> >  - New in this version.
> > ---
> > NB: Paul suggested to name the function field fn instead of func, so
> > that it would match the naming of the devfn field. Sadly the func
> > field cannot be aliased to another field using a union because it's a
> > bit field, so the only option is to rename func to fn.
> 
> Is that true? Can you not do something like...
> 
> union {
>   struct {
>     uint8_t func : 3,
>             dev  : 5;
>   };
>   struct {
>     uint8_t fn   : 3,
>             pad  : 5;
>   }; 
>   uint8_t  extfunc;
> };
> 
> ? I'm certainly not seeing any complaints from gcc.

D'oh, right. I was so focused on aliasing the func field only that it
didn't occur to me. I could even do:

union {
    struct {
        uint8_t func : 3,
                dev  : 5;
    };
    struct {
        uint8_t fn : 3,
                   : 5;
    };
    uint8_t extfunc,
            devfn;
};

Let me make sure this works with all compilers and I will send an
updated version of this patch.

Thanks, Roger.

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