Re: [virtio] Re: [virtio-comment] [PATCH v2] introduction: document #define syntax

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Apr 08, 2021 at 10:18:13PM +0200, Halil Pasic wrote:
> On Thu, 18 Mar 2021 14:23:33 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > We use the C #define syntax to refer to numeric values.
> > Let's document that.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > 
> > changes from v1:
> > address Cornelia's comments
> > 
> > 
> >  introduction.tex | 23 +++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> > 
> > diff --git a/introduction.tex b/introduction.tex
> > index cc38e29..47bebf5 100644
> > --- a/introduction.tex
> > +++ b/introduction.tex
> > @@ -210,6 +210,29 @@ \section{Structure Specifications}
> >  \begin{lstlisting}
> >  CPU_TO_BE16(B << 15 | A)
> >  \end{lstlisting}
> > +\section{Constant Specifications}
> > +
> > +In many cases, numeric values used in the interface between the device
> > +and the driver are documented using the C #define and /* */
> > +comment syntax. Multiple related values are grouped together with
> > +a common name as a prefix, using _ as a separator.
> > +Using _XXX as a suffix refers to all values in a group.
> > +For example:
> > +
> > +\begin{lstlisting}
> > +/* Field Fld value A description */
> > +#define VIRTIO_FLD_A        (1 << 0)
> > +/* Field Fld value B description */
> > +#define VIRTIO_FLD_B        (1 << 1)
> > +\end{lstlisting}
> > +documents two numeric values for a field \field{Fld}, with
> > +\field{Fld} having value 1 referring to \field{A} and \field{Fld}
> > +having value 2 referring to \field{B}.
> > +Note that $<<$ refers to the shift-left operation.
> > +
> > +Further, in this case VIRTIO_FLD_A and VIRTIO_FLD_B
> > +refer to values 1 and 2 of Fld respectively. Further, VIRTIO_FLD_XXX refers to
> > +either VIRTIO_FLD_A or VIRTIO_FLD_B.
> 
> IMHO we also use the define stuff to do flags, and that does not match
> nicely with the above sentence, since VIRTIO_FLD_F_A does not mean that
> the FLD_F field has value A if we are trying to specify a flag or a
> feature bit. And the left-shift is actually most common in these cases.
> 
> I'm not opposed to this change. In the virtio spec we tend to not be
> very rigorous about formalism, and I came to accept that as a given.
> Just wanted to point it out.
> 
> Sorry for not commenting on this sooner.
> 
> Regards,
> Halil


All true. Do you want to propose an enhancement, or just let it be for
now?

> >  
> >  \newpage
> >  
> 
> 
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