Subject: Re: [PATCH net-next] packet: doc: how to PACKET_MMAP with one packet socket for rx and tx

From: Daniel Borkmann <[email-masked]>
Date: Wed, 08 Jan 2014 15:29:38 +0100

On 01/08/2014 03:23 PM, Norbert van Bolhuis wrote:
>
> Describe how to use one AF_PACKET socket for rx and tx.
>
> Cc: Daniel Borkmann <[email-masked]>
> Signed-off-by: Norbert van Bolhuis <[email-masked]>
> ---
>   Documentation/networking/packet_mmap.txt |   14 ++++++++++++++
>   1 files changed, 14 insertions(+), 0 deletions(-)
>
> diff --git a/Documentation/networking/packet_mmap.txt b/Documentation/networking/packet_mmap.txt
> index 723bf3d..2503a5a 100644
> --- a/Documentation/networking/packet_mmap.txt
> +++ b/Documentation/networking/packet_mmap.txt
> @@ -98,6 +98,11 @@ by the kernel.
>   The destruction of the socket and all associated resources
>   is done by a simple call to close(fd).
>
> +As without PACKET_MMAP, it is possible to use one socket for
> +capture and transmission. This can be done by mapping the
> +allocated RX and TX buffer ring with a single mmap() call. See
> +"Mapping and use of the circular buffer (ring)".
> +
>   Next I will describe PACKET_MMAP settings and its constraints,
>   also the mapping of the circular buffer in the user process and
>   the use of this buffer.
> @@ -414,6 +419,15 @@ tp_block_size/tp_frame_size frames there will be a gap between
>   the frames. This is because a frame cannot be spawn across two
>   blocks.
>
> +To use one socket for capture and transmission, the mapping of both the RX
> +and TX buffer ring has to be done with one call to mmap:

As this caused some confusion in the past for some people, it would be
great to mention here in this example also:

        ...
        setsockopt(fd, SOL_PACKET, PACKET_RX_RING, &foo, sizeof(foo));
        setsockopt(fd, SOL_PACKET, PACKET_TX_RING, &bar, sizeof(bar));
        ...
> +    rx_ring = mmap(0, size * 2, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
> +    tx_ring = rx_ring + size;
> +
> +RX must be the first as the kernel maps the TX ring memory right after
> +the RX one.

Otherwise looks perfect!

>   At the beginning of each frame there is an status field (see
>   struct tpacket_hdr). If this field is 0 means that the frame is ready
>   to be used for the kernel, If not, there is a frame the user can read