Em Wed, 6 May 2020 03:55:48 -0300
"Daniel W. S. Almeida" <dwlsalme...@gmail.com> escreveu:
> Hi Mauro,
>
>
> > As commented, don't use WARN_ON(). At most, you could use WARN_ON_ONCE(),
> > as otherwise, you may end by causing serious performance issues if
> > the code starts to produce a flood of warnings at the dmesg.
> >
> > I would use pr_warn_ratelimit() on all such cases.
> >
>
> OK.
>
>
>
>
> > I don't like the idea of changing the "from" buffer endiannes, copy
> > and then restore it back to the original state. Is this really needed?
> >
> > I would, instead, define:
> >
> > struct pes_header {
> > ...
> > __be32 bitfield;
> > __be16 length;
> > ...
> > };
> >
> > Then wherever you would touch them:
> >
> > u32 bitfield;
> > u16 len;
> >
> > /* Write into BE fields */
> > pes_header.bitfield = cpu_to_be32(bitfield);
> > pes_header.length = cpu_to_be16(len);
> >
> > /* Read from BE fields */
> > bitfield = be32_to_cpu(pes_header.bitfield);
> > len = be16_to_cpu(pes_header.length);
> >
> >
> > As a side effect, when you use "__be16" and "__be32" types, gcc
> > and smatch/sparse will warn you if you mess with endiannes.
> >
> > Same applies to similar code elsewhere.
> >
>
> I don't like it either, it is error prone. I did not know about this
> other possibility. Does this work for _bitfields_ though?
See my comment below.
> I think the authors for libdvbv5 used unions precisely so bswap() could
> be called on a 'bitfield' member and from then on the bitfields could be
> accessed directly, e.g.:
>
> union {
> u16 bitfield; <-- call bswap() on this
> struct {
> --> then use these directly:
> u8 syntax:1;
> u8 zero:1;
> u8 one:2;
> u16 section_length:12;
> } __packed;
> } __packed
>
> At least that's what I understood.
You should double-check the structs from the specs. If I'm not mistaken,
bytes were swapped on some places. As I commented for patch 08/11,
the focus there were to make life simpler for userspace, and not to
store a precise copy of the byte order.
>
> I found this:
> https://lwn.net/Articles/741762/
>
> Maybe *_get_bits, *_replace_bits is the equivalent that I should use for
> bitfields?
I never used them, but, based on their definition:
static __always_inline base type##_get_bits(__##type v, base field) \
{ \
return (from(v) & field)/field_multiplier(field); \
}
Calling be16_get_bits should do the right cast to the type.
I don't know what the "from()" and "to()" macros would do.
I guess you will need to do some tests to see if this works as
expected.
>
> Because I'd rather not do this:
>
> > u32 bitfield;
> > /* Write into BE fields */
> > pes_header.bitfield = cpu_to_be32(bitfield);
>
> Since I'd have to write the (many!) bitwise operations myself and I'm
> sure I will mess this up at _some_ point.
If you mess up, gcc (and/or smatch) will complain. I mean,
if bitfield is declared as __be32, if you do:
u32 bitfield;
pes_header.bitfield = bitfield;
this will produce warnings.
Thanks,
Mauro
