[alsa-devel] [Xen-devel][PATCH 0/2] sndif: add explicit back and front synchronization

[Takashi Iwai]

On Tue, 06 Mar 2018 14:30:05 +0100,
Oleksandr Andrushchenko wrote:
> 
> On 03/06/2018 02:52 PM, Takashi Iwai wrote:
> > On Tue, 06 Mar 2018 13:05:16 +0100,
> > Oleksandr Andrushchenko wrote:
> >> On 03/06/2018 01:32 PM, Takashi Iwai wrote:
> >>> On Tue, 06 Mar 2018 12:25:07 +0100,
> >>> Oleksandr Andrushchenko wrote:
> >>>> On 03/06/2018 12:52 PM, Takashi Iwai wrote:
> >>>>> On Mon, 05 Feb 2018 09:24:58 +0100,
> >>>>> Oleksandr Andrushchenko wrote:
> >>>>>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko at epam.com>
> >>>>>>
> >>>>>> Hi, all!
> >>>>>>
> >>>>>> Foreword
> >>>>>> ========
> >>>>>>
> >>>>>> This change is aimed to add support for explicit back and front
> >>>>>> synchronization during playback and capture in response to comments
> >>>>>> raised during upstream attempt of the para-virtualized sound frontend
> >>>>>> driver for Xen [1], [2] and gather opinions from the relevant communities
> >>>>>> (ALSA, Xen) on the change.
> >>>>>>
> >>>>>> The relevant backend is implemented as a user-space application [3]
> >>>>>> and uses accompanying helper library [4].
> >>>>>>
> >>>>>> Both frontend driver and backend were tested on real HW running Xen hypervisor
> >>>>>> (Renesas R-Car ARM based H3/M3 boards, x86) to make sure the proposed
> >>>>>> solution does work.
> >>>>>>
> >>>>>> Rationale
> >>>>>> =========
> >>>>>>
> >>>>>> During the first attempt to upstream the Linux front driver [5] number
> >>>>>> of comments and concerns were raised, one of the biggest flaws in the
> >>>>>> design were questioned by both Clemens Ladisch [6] and
> >>>>>> Takashi Sakamoto [7]: the absence of synchronization between frontend
> >>>>>> and backend during capture/playback. Two options were discussed:
> >>>>>>
> >>>>>> “In design of ALSA PCM core, drivers are expected to synchronize to
> >>>>>> actual hardwares for semi-realtime data transmission. The
> >>>>>> synchronization is done by two points:
> >>>>>> 1) Interrupts to respond events from actual hardwares.
> >>>>>> 2) Positions of actual data transmission in any serial sound interfaces
> >>>>>>        of actual hardwares.
> >>>>>> “
> >>>>>>
> >>>>>> and finally a change to the existing protocol was suggested:
> >>>>>>
> >>>>>> “In 'include/xen/interface/io/sndif.h', there's no functionalities I
> >>>>>> described the above:
> >>>>>> 1. notifications from DomU to Dom0 about the size of period for
> >>>>>>        interrupts from actual hardwares. Or no way from Dom0 to DomU about
> >>>>>>        the configured size of the period.
> >>>>>> 2. notifications of the interrupts from actual hardwares to DomU.”
> >>>>>>
> >>>>>> This is implemented as a change to the sndif protocol and allows removing
> >>>>>> period emulation:
> >>>>>> 1. Introduced a new event channel from back to front
> >>>>>> 2. New event with number of bytes played/captured (XENSND_EVT_CUR_POS,
> >>>>>>       to be used for sending snd_pcm_period_elapsed at frontend (in Linux
> >>>>>>       implementation). Sent in bytes, not frames to make the protocol
> >>>>>>       generic and consistent)
> >>>>>> 3. New request for playback/capture control (XENSND_OP_TRIGGER) with
> >>>>>>       start/pause/stop/resume sub-ops
> >>>>>> 4. Playback/capture buffer size is set on the backend side via
> >>>>>>       XENSND_FIELD_BUFFER_SIZE XenStore entry
> >>>>> So the new addition looks serving well for the point that was
> >>>>> suggested in the previous thread.  As I see no frontend driver
> >>>>> implementation, it's hard to tell about the details, but through a
> >>>>> quick glance, the protocol should be OK.
> >>>> Thank you, the driver is at [1]
> >>>>> Now, going back to a big picture: I took a look at the previous
> >>>>> patchset, and wonder what about the hw_params setup.  Basically the
> >>>>> (frontend) application may request any size of buffer and periods
> >>>>> unless the driver sets up the hw constraints at open callback.  That
> >>>>> is, app may request even the 16 bytes of buffer size, or 1GB of
> >>>>> buffer.  The periods aren't always integer, so it can be 1024 bytes of
> >>>>> buffer with 400 bytes of periods.
> >>>>>
> >>>>> And, if such parameters are set up freely in the frontend side, how
> >>>>> the backend is supposed to behave?  From the frontend POV, it expects
> >>>>> receiving the wakeup/notification at each period processing (e.g. 400
> >>>>> bytes in the case above).  But, the backend is another application, so
> >>>>> how would it work for such requirements?  Am I missing something here?
> >>>> Well, the frontend is not that free to decide as it might look like,
> >>>> e.g. please see [2]. Basically part of hw_params configuration is written
> >>>> to XenStore [3] as a part of domain configuration which depends on
> >>>> system/backend
> >>>> capabilities. E.g., we usually set buffer sizes to match real HW at
> >>>> backend side
> >>>> if we use ALSA and we have more freedom if we use PulseAudio there.
> >>>> Finally, if backend decides that the requested buffer/period sizes are
> >>>> not acceptable it will reject such a configuration.
> >>> OK, that restricts minimally.  So at least there is the restriction /
> >>> communication about the buffer size.  But it merely means the
> >>> *maximum* buffer size is set.  Application may request still any
> >>> shorter value than that.
> >>>
> >>> And, there are no restriction about period sizes (except for the
> >>> periods_max, which is calculated from buffer_bytes_max).
> >>> That is, application may request any size between them; and it expects
> >>> the wake up by this value.
> >>>
> >>> I think that's a still missing stone in the design.
> >> Well, so what would a real HW driver do in that case?
> >> My understanding is that in this case SW can still request
> >> something that HW can't do and driver will reject such configurations.
> >> In my case, the role of that HW driver code which judges on if configuration
> >> is acceptable just runs on the backend side, e.g. frontend driver is just
> >> a proxy which talks to the backend to check if the backend can do what
> >> requested.
> >> And it is up to backend to decide.
> >>
> >> Does that sound reasonable or you have something else on your mind?
> > Usually the hardware driver knows already the restrictions and sets up
> > the rules via hw constraints at open callback.  There are lots of
> > snd_pcm_hw_constraint_*() helpers (and the relevant ones) to give more
> > additional rules for the parameter restrictions.  For example, if the
> > periods must be aligned with the buffer size (i.e. buffer_size %
> > period_size == 0) as on many devices, you can call like:
> >    snd_pcm_hw_constraint_integer(substream->runtime,
> > 	SNDRV_PCM_HW_PARAM_PERIODS);
> > in the open callback.
> You are right, I saw those in other drivers
> > And, now an open question for XEN comes: what kind of restriction
> > should be applied to the frontend.  Obviously it depends on the
> > backend, so there must be some communication, and the restriction must
> > be propagated at open, i.e. *before* actually hw_params is performed.
> Could you please give me a hint of what those restrictions
> could look like? E.g. map of supported buffer/period sizes, what else?

Heh, that very much depends on the hardware -- and in this case, on
the implementation of the backend.

Practically seen, the buffer and the period size setups are mandatory,
yes.  Here is the question whether you want to limit them by list
(e.g. read via some XENSND_* protocol), or negotiate the size at each
hw_params setup (e.g. getting only min/max at open, and at each
hw_params call, negotiate with the backend for period and buffer size
changes).

The format, the channels and the sample rate are already included in
snd_pcm_hardware setup, so this should be OK, unless they have
implicit limitations with each other (e.g. some format is available
only under some rate).

Maybe the channels need to be revisited, though; usually you can't
handle all number of channels between min and max but only even
numbers or such.


thanks,

Takashi