On Thu, Jul 23, 2015 at 08:11:11AM +0200, Daniel Mack wrote:
> On 07/23/2015 03:00 AM, Peter Chen wrote:
> >> That detail is merely about completeness. The code that calculates the
> >> > value of wMaxPacketSize should take into account what is configured in
> >> > bInterval of the endpoint, so if users change one thing, they don't have
> >> > to tweak the other as well.
> >> >
> >> > bInterval denotes how many packets an endpoint can serve per second, and
> >> > wMaxPacketSize defines how large each packet can be. So in an
> >> > application that knows how many bytes/s are to be transferred,
> >> > wMaxPacketSize depends on bInterval.
> >> >
> >> > On HS endpoints, we have 8 microframes per USB frame, so the divisor is
> >> > 8000, not 1000. However, I just figured the descriptors in f_uac2 set
> >> > .bInterval to 4, which means a period of 8 (2^(4-1)), and that
> >> > compensates the factor again.
> >> >
> >> > So, to conclude - your calculation indeed comes up with the correct
> >> > value, but it should still take the configured endpoint details into
> >> > account so the code makes clear how the numbers are determined.
> >> > Something like the following should work:
> >> >
> >> > /* for FS */
> >> > div = 1000 / (1 << (fs_epout_desc->bInterval - 1));
> >> >
> >> > /* for HS */
> >> > div = 8000 / (1 << (hs_epout_desc->bInterval - 1));
> >> >
> >> > c_max_packet_size = uac2_opts->c_chmask * uac2_opts->c_ssize
> >> > * DIV_ROUND_UP(uac2_opts->c_srate, div);
> >> >
> >> >
> >> > Makes sense?
> >> >
> > Thanks, it is correct. But looking the code at afunc_set_alt:
> > the method of calculating uac2->p_pktsize seems incorrect, it
> > may need to change like below:
> >
> > @@ -1176,15 +1188,16 @@ afunc_set_alt(struct usb_function *fn, unsigned intf, unsigned alt)
> > factor = 1000;
> > } else {
> > ep_desc = &hs_epin_desc;
> > - factor = 125;
> > + factor = 8000;
> > }
> >
> > /* pre-compute some values for iso_complete() */
> > uac2->p_framesize = opts->p_ssize *
> > num_channels(opts->p_chmask);
> > rate = opts->p_srate * uac2->p_framesize;
> > - uac2->p_interval = (1 << (ep_desc->bInterval - 1)) * factor;
> > - uac2->p_pktsize = min_t(unsigned int, rate / uac2->p_interval,
> > + uac2->p_interval = factor / (1 << (ep_desc->bInterval - 1));
> > + uac2->p_pktsize = min_t(unsigned int, DIV_ROUND_UP(rate,
> > + uac2->p_interval),
> > prm->max_psize);
>
> Your p_interval calculation is equivalent in both cases:
>
> FS: 1 * 1000 == 1000 / 1
> HS: 8 * 125 == 8000 / 8
>
> And no, p_pktsize is intentionally set to the minimum packet size that a
> packet will usually have. The actual size might be higher due to the
> accumulated residue (see below).
>
Assume the interval for each packet is 2ms, the rate is 192 kbytes
for both FS & HS:
uac2->p_interval = 2000;
uac2->p_pktsize = 192 kbytes / 2000 = 96 bytes
And the uac2->p_pktsize is real usb packet length, it means hardware
would expect there are 96 bytes per 2ms, but the real frame rate is 192k,
and it needs to 192 * 2 bytes per 2ms in the bus, am I missing
something?
Another think I am not understand is why playback uses IN endpoint?
Don't this playback mean the data is from host, and play at device side?
--
Best Regards,
Peter Chen
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