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

[Konrad Rzeszutek Wilk]

>   * +----------------+----------------+----------------+----------------+
>   * |                           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.