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Date:   Fri, 15 Mar 2019 10:24:28 -0700
From:   Stanislav Fomichev <sdf@fomichev.me>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc:     Eric Dumazet <edumazet@google.com>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCH] tcp: don't use __constant_cpu_to_be32
Message-ID: <20190315172428.GB5481@mini-arch.hsd1.ca.comcast.net>
References: <20190314061514.19586-1-sergey.senozhatsky@gmail.com>
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 <20190315012012.GA748@jagdpanzerIV>
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On 03/15, Sergey Senozhatsky wrote:
> On (03/14/19 05:42), Eric Dumazet wrote:
> > > cpu_to_be32() is capable enough to detect __builtin_constant_p()
> > > and to use an appropriate compile time ___constant_swahb32()
> > > function.
See my recent commit a0517a0f7ef23 ("selftests/bpf: use
__bpf_constant_htons in test_prog.c") where compiler was not smart
enough to figure it out (you can google similar issues for GCC < 4.8).

> > >
> > > So we can use cpu_to_be32() instead of __constant_cpu_to_be32().
> > 
> > 
> > I dunno, this is uapi, this might break user space for some funky compiler ?
> > I do not even know if cpu_to_be32() _is_ uapi.
> 
> Hmm, excellent point.
> 
> Looking at
> 
> if_pppox.h:#define PTT_EOL		__cpu_to_be16(0x0000)
> if_pppox.h:#define PTT_SRV_NAME	__cpu_to_be16(0x0101)
> if_pppox.h:#define PTT_AC_NAME	__cpu_to_be16(0x0102)
> if_pppox.h:#define PTT_HOST_UNIQ	__cpu_to_be16(0x0103)
> if_pppox.h:#define PTT_AC_COOKIE	__cpu_to_be16(0x0104)
> if_pppox.h:#define PTT_VENDOR 	__cpu_to_be16(0x0105)
> if_pppox.h:#define PTT_RELAY_SID	__cpu_to_be16(0x0110)
> if_pppox.h:#define PTT_SRV_ERR     __cpu_to_be16(0x0201)
> if_pppox.h:#define PTT_SYS_ERR  	__cpu_to_be16(0x0202)
> if_pppox.h:#define PTT_GEN_ERR  	__cpu_to_be16(0x0203)
> if_tunnel.h:#define GRE_CSUM	__cpu_to_be16(0x8000)
> if_tunnel.h:#define GRE_ROUTING	__cpu_to_be16(0x4000)
> if_tunnel.h:#define GRE_KEY		__cpu_to_be16(0x2000)
> if_tunnel.h:#define GRE_SEQ		__cpu_to_be16(0x1000)
> if_tunnel.h:#define GRE_STRICT	__cpu_to_be16(0x0800)
> if_tunnel.h:#define GRE_REC		__cpu_to_be16(0x0700)
> if_tunnel.h:#define GRE_ACK		__cpu_to_be16(0x0080)
> if_tunnel.h:#define GRE_FLAGS	__cpu_to_be16(0x0078)
> if_tunnel.h:#define GRE_VERSION	__cpu_to_be16(0x0007)
> if_tunnel.h:#define GRE_VERSION_0		__cpu_to_be16(0x0000)
> if_tunnel.h:#define GRE_VERSION_1		__cpu_to_be16(0x0001)
> if_tunnel.h:#define GRE_PROTO_PPP		__cpu_to_be16(0x880b)
> 
> You are right, probably would need __cpu_to_be32, which also does
> the constant folding.
> 
> =-=-=-=-=
> 
> Subject: [PATCH] tcp: don't use __constant_cpu_to_be32
> 
> __cpu_to_be32() can detect __builtin_constant_p() at compile time.
> So we can use __cpu_to_be32() instead of __constant_cpu_to_be32().
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  include/uapi/linux/tcp.h | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
> index 8bb6cc5f3235..cf9c246a59c8 100644
> --- a/include/uapi/linux/tcp.h
> +++ b/include/uapi/linux/tcp.h
> @@ -70,16 +70,16 @@ union tcp_word_hdr {
>  #define tcp_flag_word(tp) ( ((union tcp_word_hdr *)(tp))->words [3]) 
>  
>  enum { 
> -	TCP_FLAG_CWR = __constant_cpu_to_be32(0x00800000),
> -	TCP_FLAG_ECE = __constant_cpu_to_be32(0x00400000),
> -	TCP_FLAG_URG = __constant_cpu_to_be32(0x00200000),
> -	TCP_FLAG_ACK = __constant_cpu_to_be32(0x00100000),
> -	TCP_FLAG_PSH = __constant_cpu_to_be32(0x00080000),
> -	TCP_FLAG_RST = __constant_cpu_to_be32(0x00040000),
> -	TCP_FLAG_SYN = __constant_cpu_to_be32(0x00020000),
> -	TCP_FLAG_FIN = __constant_cpu_to_be32(0x00010000),
> -	TCP_RESERVED_BITS = __constant_cpu_to_be32(0x0F000000),
> -	TCP_DATA_OFFSET = __constant_cpu_to_be32(0xF0000000)
> +	TCP_FLAG_CWR = __cpu_to_be32(0x00800000),
> +	TCP_FLAG_ECE = __cpu_to_be32(0x00400000),
> +	TCP_FLAG_URG = __cpu_to_be32(0x00200000),
> +	TCP_FLAG_ACK = __cpu_to_be32(0x00100000),
> +	TCP_FLAG_PSH = __cpu_to_be32(0x00080000),
> +	TCP_FLAG_RST = __cpu_to_be32(0x00040000),
> +	TCP_FLAG_SYN = __cpu_to_be32(0x00020000),
> +	TCP_FLAG_FIN = __cpu_to_be32(0x00010000),
> +	TCP_RESERVED_BITS = __cpu_to_be32(0x0F000000),
> +	TCP_DATA_OFFSET = __cpu_to_be32(0xF0000000)
>  }; 
>  
>  /*
> -- 
> 2.21.0
>