Re: [RTnet-developers] SO_TIMESTAMPNS for RTnet

[Jan K. <jan...@we...>]

On 2013-05-19 10:24, Manuel Huber wrote:
> Hello!
> 
> I'm currently working on a patch for RTnet
> (https://bitbucket.org/robinreal/rtnetts/overview exp branch) that
> enables the user to retrieve time stamps from the driver (via
> recvmsg). Currently development is not finished, there are still
> bugs and I need to rewrite commit messages. Overview:
> 
> * I hooked into the rt_udp_recvmsg function to optionally add time
>    stamps to the msghdr struct. The rt_socket_recv_timestamp function
>    is used to actually retrieve put the time stamp into the cmsg
>    buffer. This could maybe also be used for other protocols (?).
> 
> * In stack/scm.c the original put_cmsg function has been ported to
>    RTDM. Currently I haven't yet implemented the put_cmsg_compat
>    function.
> 
> * In rtnet_socket.h I added a member sk_flags, which could be used for
>    various other functions. This is used to activate and deactivate
>    passing time stamps to the user (through msghdr structure).
> 
> * CONFIG_RTNET_SO_TIMESTAMPNS can be used to control whether RTnet
>    stack supports passing timestamps to user-space or not.

Still need to study this. Will have a look these days while traveling.

> 
> 
> During implementation I ran into some problems:
> 
> * I couldn't really find out, where the "normal" Linux Kernel fixes
>    the cmsg structure. After all messages have been added to the
>    buffer, the original start address and the resulting length has to
>    be written to the structure. I'm currently doing this in
>    rt_udp_recvmsg (just before "return") but I believe there is a
>    better place...

You could run the kernel against a debugger (kgdb or the built-in
gdbserver of QEMU/KVM) and set a watchpoint...

> 
> * I don't understand how to implement socket options. Is it okay to
>    just introduce a new ioctl, or is it better to use setsockopt? I'm
>    just unsure where to implement this. Currently I wrote some code to
>    ip_sock.c in stack/ipv4 but that's wrong. It currently doesn't work
>    because of the level... The Linux Kernel has got the file sock.c
>    which implements a handler for SOL_SOCKET. Is there a simple method
>    to use setsockopt, or is it better to just introduce a new ioctl?

You receive setsockopt request via an official IOCTL from the RTDM
layer. Watch out for _RTIOC_SETSOCKOPT.

> 
> * Is it a good idea to support a similar interface as normal
>    SO_TIMESTAMPNS which requires to transform nanosecs_abs_t to a
>    timespec structure?  To support this, I would have to evaluate a
>    division and a modulo operation of a 64bits variable and a 32bit
>    constant. I'm not sure whether this is a good idea or not. Currently
>    I'm thinking about inventing a new cmsg identifier and flag to just
>    copy the nanosecs variable and also support SO_TIMESTAMPNS (which
>    will introduce some extra latency when active).

On x86, the conversion will be barely visible. But on low-end archs, it
may make a little difference. However, supporting a standard interface
makes some sense to ease portability, so you should have SO_TIMESTAMPNS.
An optimized interface could be added on demand.

Jan