From mboxrd@z Thu Jan  1 00:00:00 1970
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Subject: Re: [Xen-devel][PATCH 2/2] sndif: add explicit back
 and front synchronization
Date: Thu, 1 Mar 2018 17:11:56 -0500
Message-ID: <20180301221156.GC32676@char.us.oracle.com>
References: <1517819100-1029-1-git-send-email-andr2000@gmail.com>
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To: Oleksandr Andrushchenko <andr2000@gmail.com>
Cc: alsa-devel@alsa-project.org, Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>, Takashi Iwai <tiwai@suse.de>, Clemens Ladisch <clemens@ladisch.de>, Takashi Sakamoto <o-takashi@sakamocchi.jp>, Oleksandr Grytsov <oleksandr_grytsov@epam.com>, xen-devel@lists.xenproject.org
List-Id: alsa-devel@alsa-project.org

>   * +----------------+----------------+----------------+----------------+
>   * |                           gref_directory                          | 24
>   * +----------------+----------------+----------------+----------------+
> - * |                             reserved                              | 28
> - * +----------------+----------------+----------------+----------------+
> - * |/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/|
> + * |                             period_sz                             | 28
>   * +----------------+----------------+----------------+----------------+
>   * |                             reserved                              | 32
>   * +----------------+----------------+----------------+----------------+
> @@ -578,6 +616,14 @@
>   * pcm_channels - uint8_t, number of channels of this stream,
>   *   [channels-min; channels-max]
>   * buffer_sz - uint32_t, buffer size to be allocated, octets
> + * period_sz - uint32_t, recommended event period size, octets
> + *   This is the recommended (hint) value of the period at which frontend would
> + *   like to receive XENSND_EVT_CUR_POS notifications from the backend when
> + *   stream position advances during playback/capture.
> + *   It shows how many octets are expected to be played/captured before
> + *   sending such an event.
> + *   If set to 0 no XENSND_EVT_CUR_POS events are sent by the backend.
> + *

I would gate this based on the version. That is if version 0 then this
field does not exist.
>   * gref_directory - grant_ref_t, a reference to the first shared page
>   *   describing shared buffer references. At least one page exists. If shared
>   *   buffer size  (buffer_sz) exceeds what can be addressed by this single page,
> @@ -592,6 +638,7 @@ struct xensnd_open_req {
>      uint16_t reserved;
>      uint32_t buffer_sz;
>      grant_ref_t gref_directory;
> +    uint32_t period_sz;

The same here. Just put a comment mentioning the version part.