Re: [Xen-devel] [PATCH RESEND1 00/12] ALSA: vsnd: Add Xen para-virtualized frontend driver

[Takashi Sakamoto <o-takashi@xxxxxxxxxxxxx>]

Hi,

On Aug 7 2017 21:22, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>
> 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,
> Oleksandr
>
> 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.

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.

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. 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.
2. notifications of the interrupts from actual hardwares to DomU.

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.

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.


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. 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].
 * 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
   upstream[4].
 * 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
   done.
 * In my taste, a prefix of the subject line should be 'xen-front',
  instead of 'vsnd'. It comes from name of your driver.

[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

Regards

Takashi Sakamoto

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