Re: [PATCH] firewire: adopt read cycle timer ABI from raw1394

["Kristian Høgsberg" <krh@bitplanet.net>]

On 10/1/07, Pieter Palmers <pieterp@joow.be> wrote:
> Stefan Richter wrote:
> >> This duplicates the read cycle timer feature of raw1394 (added in Linux
> >> 2.6.21) in firewire-core's userspace ABI.
> >
> > Kristian and Pieter, does this simple duplication of the ioctl make
> > sense on its own?  AFAIU rawiso's iso packet buffers look different from
> > fw-cdevs's. It seems to me as if rawiso always put the cycle into a user
> > buffer for each iso packet received...
> >
> > raw1394.h::struct raw1394_iso_packet_info {
> >       __u32 offset;
> >       __u16 len;
> >       __u16 cycle;   /* recv only */
> >       __u8  channel; /* recv only */
> >       __u8  tag;
> >       __u8  sy;
> > };
> >
> > raw1394.c::raw1394_iso_recv_packets()
> >
> >       /* copy the packet_infos out */
> >       for (i = 0; i < upackets.n_packets; i++) {
> >               if (__copy_to_user(&upackets.infos[i],
> >                                  &fi->iso_handle->infos[packet],
> >                                  sizeof(struct raw1394_iso_packet_info)))
> >                       return -EFAULT;
> >
> >               packet = (packet + 1) % fi->iso_handle->buf_packets;
> >       }
> >
> > ...while the Juju ABI returns the cycle only for those packets whose
> > fw_cdev_iso_packet.control had the FW_CDEV_ISO_INTERRUPT flag set.
> > The cycle is then written out in the fw_cdev_event_iso_interrupt event
> > which happens when this particular packet was received.  Right?
> >
> > Pieter, do applications like yours need the cycle counter only for a few
> > predetermined packets or for each and every packet?
>
> We need it for every packet for two reasons:
> 1) it's the only way to determine how many packets were dropped when
> packet drops are flagged in the callback

Your application should know what the timestamp should be for each iso
receive callback and if you see a larger value than expected you can
use that to calculate how many cycles were lost.

> 2) we convert the 16-bit SYT timestamp of a packet to a full 32-bit
> cycle counter value. This because the range of the 16-bit SYT is too
> small (only 16 packets) for systems that have large buffering.

If you get the timestamp for the first packet in a receive batch, you
can still do this, right?

cheers,
Kristian