Messages in this thread | | | Date | Wed, 6 May 2020 03:55:48 -0300 | From | "Daniel W. S. Almeida" <> | Subject | Re: [RFC, WIP, v4 09/11] media: vidtv: implement a PES packetizer |
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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?
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.
I found this: https://lwn.net/Articles/741762/
Maybe *_get_bits, *_replace_bits is the equivalent that I should use for bitfields?
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.
thanks, - Daniel
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