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

In short: yes we use it for every packet.

Pieter
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