[pulseaudio-discuss] [PATCH v3] bluetooth: Wide Band Speech implementaion for backend ofono

[Sathish Narasimman]

Hi James,

On Mon, Aug 20, 2018 at 8:19 PM James Bottomley <
James.Bottomley at hansenpartnership.com> wrote:

> On Mon, 2018-08-20 at 19:19 +0530, Sathish Narasimman wrote:
> > From: Sathish Narasimman <sathish.narasimman at intel.com>
> >
> > mSBC-encoded streams for HFP. The Wide Band Speech(WBS) encoding and
> > decoding
> > is implemeted with this patch. This patch was refered from original
> > patch of Joao Paula Rechi Vita and was verified with the supported
> > bluetooth controller.
>
> Which headset did you test this with?  When I try it with an LG 900 I
> get a huge amount of chop which shreds the audio quality.  For this
> headset the packet size (MTU) still needs to be set at 48 on both the
> send and receive side otherwise the audio doesn't work at all.
>
I used Jabra EasyGo GNM-OTE4 bluetooth headset.
This patch is refered w.r.t to USB transport.
For USB transport - As per core spec 5, vol 4, Part B Table 2.1(this was
the only reference in core spec for USB transport- please correct me if i
am wrong)
for msbc voice channel it should be 63bytes along with Usb header. So the
host
should send 60 bytes of msbc audio.

>
> What seems to be happening is that the LG insists on running the eSCO
> links at full speed but the encoded packets only take about half the
> bandwidth, so on the headset receiving side, we get about half as many
> mSBC encoded packets with the rest being zero padded.
>
> The problem occurs because on the receive side you count mSBC packets,
> so the reader/writer thread you try to send 1 mSBC encoded packet for
> every 1 mSBC packet you receive.
>
> But in this send loop:
>
> > +        /* Send MTU bytes of it, if there is more it will send later
> > */
> > +        while (u->msbc_info.ebuffer_start + u->write_link_mtu <= u-
> > >msbc_info.ebuffer_end) {
> > +            l = pa_write(u->stream_fd,
> > +                         u->msbc_info.ebuffer + u-
> > >msbc_info.ebuffer_start,
> > +                         u->write_link_mtu,
> > +                         &u->stream_write_type);
> > +
> > +            wrote = true;
> > +            if (l <= 0) {
> > +                pa_log_debug("Error while writing: l %d, errno %d",
> > l, errno);
> > +                break;
> > +            }
> > +
> > +            u->msbc_info.ebuffer_start += l;
> > +            if (u->msbc_info.ebuffer_start >= u-
> > >msbc_info.ebuffer_end)
> > +                u->msbc_info.ebuffer_start = u-
> > >msbc_info.ebuffer_end = 0;
> > +        }
> > +
> > +        pa_memblock_release(u->write_memchunk.memblock);
> > +
> > +        if (wrote && l < 0) {
> > +
> > +            if (errno == EINTR)
> > +                /* Retry right away if we got interrupted */
> > +                continue;
> > +
> > +            else if (errno == EAGAIN)
> > +                /* Hmm, apparently the socket was not writable, give
> > up for now */
> > +                break;
> > +
> > +            pa_log_error("Failed to write data to SCO socket: %s",
> > pa_cstrerror(errno));
> > +            ret = -1;
> > +            break;
> > +        }
> > +
> > +        if ((size_t) l != (size_t)u->write_link_mtu) {
> > +            pa_log_error("Wrote memory block to socket only
> > partially! %llu written, wanted to write %llu.",
> > +                        (unsigned long long) l,
> > +                        (unsigned long long) u->write_link_mtu);
> > +            ret = -1;
> > +            break;
> > +        }
> > +
> > +        u->write_index += (uint64_t) u->write_memchunk.length;
> > +
> > +        pa_memblock_unref(u->write_memchunk.memblock);
> > +        pa_memchunk_reset(&u->write_memchunk);
> > +
> > +        ret = 1;
> > +        break;
> > +    }
>
> Because the mtu is 48 and the mSBC encode size is 60, the transmission
> stops after 48 bytes, we now wait for a new mSBC packet to be received,
> so we miss the next eSCO transmission window and the headset fills in
> with zeros causing the packet to be chopped and the next one to be
> discarded as invalid.
>
> I suspect the only way to get mSBC to work for this type of headset is
> to count actual received packets, always to transmit full 60 byte mSBC
> packets in adjacent frames and to pad with zeros if we're not ready.
>
I tried to get the MTU negotiation from the kernel.
Though getsockopt was not implemented to provide the required MTU from the
BT kernel.
I tried to implement that part to get the right MTU from the BT kernel.
But the problem here is, the USB  transport gets the right MTU from ALT
setting.
which happens after the  successfull sco connection(exactly during accept
connection or sco_accept).
So during hf_audio_agent_transport_acquire call we are unable to get the
right MTU at this function in backend-ofono.c file.
This is the reason i had planned to hard code it for now. This patch is
purely verified with USB transport

>
> James
>
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Thansk,
Sathish N
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