thank you very much for valuable comments and your time!

On 08/10/2017 06:14 AM, Takashi Sakamoto wrote:

On Aug 7 2017 21:22, Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushche...@epam.com>

This patch series adds support for Xen [1] para-virtualized
sound frontend driver. It implements the protocol from
include/xen/interface/io/sndif.h with the following limitations:
- mute/unmute is not supported
- get/set volume is not supported
Volume control is not supported for the reason that most of the
use-cases (at the moment) are based on scenarious where
unprivileged OS (e.g. Android, AGL etc) use software mixers.

Both capture and playback are supported.

Thank you,

Resending because of rebase onto [2] + added missing patch

[1] https://xenproject.org/
[2] https://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound.git/log/?h=for-next

Oleksandr Andrushchenko (12):
   ALSA: vsnd: Introduce Xen para-virtualized sound frontend driver
   ALSA: vsnd: Implement driver's probe/remove
   ALSA: vsnd: Implement Xen bus state handling
   ALSA: vsnd: Read sound driver configuration from Xen store
   ALSA: vsnd: Implement Xen event channel handling
   ALSA: vsnd: Implement handling of shared buffers
   ALSA: vsnd: Introduce ALSA virtual sound driver
   ALSA: vsnd: Initialize virtul sound card
   ALSA: vsnd: Add timer for period interrupt emulation
   ALSA: vsnd: Implement ALSA PCM operations
   ALSA: vsnd: Implement communication with backend
   ALSA: vsnd: Introduce Kconfig option to enable Xen PV sound

  sound/drivers/Kconfig     |   12 +
  sound/drivers/Makefile    |    2 +
sound/drivers/xen-front.c | 2107 +++++++++++++++++++++++++++++++++++++++++++++
  3 files changed, 2121 insertions(+)
  create mode 100644 sound/drivers/xen-front.c

For this patchset, I have the same concern which Clemens Ladisch
denoted[1]. If I can understand your explanation about queueing between
Dom0/DomU stuffs, the concern can be described in short words; this
driver works without any synchronization to data transmission by actual
sound hardwares.

Yes, both your concerns and understanding are correct
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.

These two points comes from typical designs of actual hardwares, thus
they doesn't come from unfair, unreasonable, intrusive demands from
software side.

This clear, thank you
In design of typical stuffs on para-virtualization, Dom0 stuffs are hard
to give enough abstraction of sound hardwares in these two points for
DomU stuffs. Especially, it cannot abstract point 2) at all because the
value of position should be accurate against actual time frame, while
there's an overhead for DomU stuffs to read it. When DomU stuffs handles
the value, the value is enough past due to context switches between
Dom0/DomU. Therefore, this driver must rely on point 1) to synchronize
to actual sound hardwares.
Which will also introduce some latency too: time needed to deliver and
handle interrupt from Dom0 to DomU
Typically, drivers configure hardwares to
generate interrupts per period of PCM buffer. This means that this
driver should notify to Dom0 about the value of period size requested
by applications.

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.
Ok, then on "open" command I will pass from DomU to Dom0 an additional
parameter, period size. Then Dom0 will respond with actual period size
for DomU to use. So, this way period size will be negotiated.
Does the above look ok to you?
2. notifications of the interrupts from actual hardwares to DomU.

Ok, I will introduce an event from Dom0 to DomU to signal period elapsed.

Taking into account the fact that period size may be as small as,
say, 1ms, do you think we can/need to mangle period size in 1) on Dom0 side
to be reasonable, so we do not flood with interrupts/events from Dom0 to DomU?
Do you see any "formula" to determine that reasonable/acceptable
period limit, so both Dom0 and DomU are happy?

For the reasons, your driver used kernel's timer interface to generate
'pseudo' interrupts for the purpose. However, it depends on Dom0's
abstraction different from sound hardwares and Linux kernel's
abstraction for timer functionality. In this case, gap between 'actual'
interrupts from hardware and the 'pseudo' interrupts from a combination
of several components brings unexpected result on several situations.

You are right
I think this is defects of 'sndif' interface in Xen side. I think it
better for you to work in Xen community to improve the above interface
at first, then work for Linux stuffs.

Please see above for planned changes to the protocol

Additionally, in next time, please remind of several points below:
 * When a first patch adds an initial code for drivers, it should
   include entries for Makefile and Kconfig, so that the driver can be
   built even if it's still in an initial shape.
Will do
Each patch should be
   self-contained and should be in a shape so that developers easily run
   bisecting. In other words, your first patch[2] includes modification
  for Makefile and Kconfig in your last patch[3].
Will do
 * When any read-only symbols is added,  it should have 'const'
   qualifier so that the symbol places to .rodata section of ELF
   binaries. For example, in your code, 'alsa_sndif_formats' is such an
   symbol. In recent Linux development, some developers work for
   constifying such symbols. Please remind of their continuous works in
Will do
 * You can split your driver to several files. In
   'include/xen/interface/io/sndif.h', Dom0 produces functionalities for
   DomU to control gain/volume/mute and in future your driver may get
   more codes. If split to several files make it readable, it should be
Will do. If I split, do you think it would be better to move the driver
from sound/drivers to sound/xen folder, so all those files do not mix
with the rest?
 * In my taste, a prefix of the subject line should be 'xen-front',
  instead of 'vsnd'. It comes from name of your driver.

Will do
[1] [alsa-devel] [PATCH 08/11] ALSA: vsnd: Add timer for period interrupt emulation http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123617.html [2] [PATCH RESEND1 01/12] ALSA: vsnd: Introduce Xen para-virtualized sound frontend driver http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123654.html [3] [alsa-devel] [PATCH RESEND1 12/12] ALSA: vsnd: Introduce Kconfig option to enable Xen PV sound http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123662.html [4] You can see many posts for this; e.g. [alsa-devel] [PATCH 0/7] constify ALSA usb_device_id. http://mailman.alsa-project.org/pipermail/alsa-devel/2017-August/123564.html


Takashi Sakamoto
Thank you,

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