[PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

Johannes Berg johannes at sipsolutions.net
Wed Nov 24 19:03:24 AEDT 2021

On Tue, 2021-11-23 at 15:49 -0800, Jakub Kicinski wrote:
> On Tue, 23 Nov 2021 17:24:15 +0100 Johannes Berg wrote:
> > > (*_replace_bits() seems to be useful, though)  
> > 
> > Indeed.
> > 
> > Also as I said in my other mail, the le32/be32/... variants are
> > tremendously useful, and they fundamentally cannot be expressed with the
> > FIELD_GET() or field_get() macros. IMHO this is a clear advantage to the
> Can you elaborate?

Well, the way I see it, the only advantage of FIELD_GET() is that it
will auto-determine the type (based on the mask type.) This cannot work
if you need be/le conversions, because the be/le type annotations are
invisible to the compiler.

So obviously you could write a BE32_FIELD_GET(), but then really that's
equivalent to be32_get_bits() - note you you have to actually specify
the type in the macro name. I guess in theory you could make macros
where the type is an argument (like FIELD_GET_TYPE(be32, ...)), but I
don't see how that gains anything.

> > typed versions, and if you ask me we should get rid of the FIELD_GETand
> > FIELD_PREP entirely - difficult now, but at least let's not propagate
> > that?
> I don't see why.

Just for being more regular, in the spirit of "there's exactly one
correct way of doing it" :)


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