On Wed, May 17, 2017 at 6:27 AM, Miroslav Lichvar <mlich...@redhat.com> wrote:
> On Tue, May 16, 2017 at 06:34:38PM -0400, Willem de Bruijn wrote:
>> On Tue, May 16, 2017 at 8:44 AM, Miroslav Lichvar <mlich...@redhat.com>
>> wrote:
>> > If software timestamping is enabled by the SO_TIMESTAMP(NS) option
>> > when a message without timestamp is already waiting in the queue, the
>> > __sock_recv_timestamp() function will read the current time to make a
>> > timestamp in order to always have something for the application.
>> >
>> > However, this applies also to outgoing packets looped back to the error
>> > queue when hardware timestamping is enabled by the SO_TIMESTAMPING
>> > option.
>>
>> This is already the case for sockets that have both software receive
>> timestamps and hardware tx timestamps enabled, independent from
>> the new option SOF_TIMESTAMPING_OPT_TX_SWHW, right? If so,
>> then this behavior must remain.
>
> Even if we consider that it's not actually returning a valid TX
> timestamp and it didn't behave as documented ("Only one field is
> non-zero at any time")?
Even if it is not a very useful timestamp. Applications may read it nonetheless.
I think that the documentation is newer than the feature, and wrong in
this case.
> On the RX side this timestamp does make some sense as it could be
> viewed as a very late timestamp, correctly ordered after the HW
> timestamp,
Actually, on Rx it is equally problematic. It can easily generate out of
order timestamps. When enabling timestamping, the first packets will
have a timestamp generated at recvmsg while the following have one
generated at __netif_receive_skb_core.
> but on the TX side the order is reversed and returning a
> timestamp later than the actual transmission might break a protocol.
>
> If you don't see it as a bug fix, I think this weird interaction
> between the SO_TIMESTAMPING(NS) and SO_TIMESTAMPING options needs to
> be documented.
I agree. I don't think that it's all that useful, but it is
established behavior.
