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

[Takashi Iwai <tiwai@xxxxxxx>]

On Tue, 06 Mar 2018 17:04:41 +0100,
Oleksandr Andrushchenko wrote:
> 
> >>>> If we decide to negotiate the parameters, then it can't be done
> >>>> at .open stage as well, as at this moment we don't know stream
> >>>> parameters yet, e.g. we don't know the number of channels, PCM
> >>>> format etc., so we cannot explain to the backend what we want.
> >>>> Thus, it seems that we need to move the negotiation to .hw_params
> >>>> callback where stream properties are known. But this leaves the
> >>>> only option to ask the backend if it can handle the requested
> >>>> buffer/period and other parameters or not... This is what I do now :(
> >>> The additional parameter setup can be done via hw_constraints.  The hw
> >>> constraint is basically a function call for each parameter change to
> >>> narrow down the range of the given parameter.
> >>>
> >>> snd_pcm_hw_constraint_integer() in the above is just an example.
> >>> The actual function to adjust values can be freely written.
> >> Yes, this is clear, the question here mostly was not
> >> *how* to set the constraints, but *where* to get those
> > ... and here comes the hw constraint into the play.
> >
> > For each parameter change, for example, the frontend just passes the
> > inquiry to the backend.  The basis of the hw constraint is nothing but
> > to reduce the range of the given parameter.  It's either interval
> > (range, used for period/buffer size or sample rate) or the list (for
> > the format).  When any parameter is changed, ALSA PCM core invokes the
> > corresponding hw constraint function, and the function reduces the
> > range.  It's repeated until all parameters are set and settled down.
> >
> > So, for your driver, the frontend just passes the hw constraint for
> > each of basic 5 parameters to the backend.  For example, at beginning,
> > the hw constraint for the buffer size will pass the range (1,INTMAX).
> > Then the backend returns the range like (1024,65536).   This already
> > gives users the min/max buffer size information.  The similar
> > procedure will be done for all other parameters.
> >
> > In addition, you can put the implicit rule like the integer periods,
> > which makes things easier.
> >
> Thank you very much for such a detailed explanation.
> Could you please give me an example of ALSA driver which
> code I can read in order to understand how it is supposed
> to be used, e.g. which meets the expectations we have for
> Xen PV sound driver?

This is the most difficult part, apparently :)
There are lots of codes deploying the own hw constraints, but nothing
is similar like your case.

Suppose that we negotiate from the frontend to the backend like

        int query_hw_param(int parm, int *min_p, int *max_p);

so that you can call like
        err = query_hw_param(PARM_RATE, &min_rate, &max_rate);

This assumes that min_rate and max_rate were already filled by the
values requested from frontend user-space.  In query_hw_parm, the
backend receives this range, checks it, and fills again the actually
applicable range that satisfies the given range in return.

In that way, user-space will reduce the configuration space
repeatedly.  And at the last step, the configurator chooses the
optimal values that fit in the given configuration space.

As mentioned in the previous post, in your driver at open, you'd need
to add the hw constraint for each parameter.  That would be like:

        err = snd_pcm_hw_rule_add(runtime, 0, SNDRV_PCM_HW_PARAM_RATE,
                                  hw_rule_rate, NULL, -1);

and hw_rule_rate() would look like:

static int hw_rule_rate(struct snd_pcm_hw_params *params,
                        struct snd_pcm_hw_rule *rule)
{
        struct snd_interval *p =
                hw_param_interval(params, SNDRV_PCM_HW_PARAM_RATE);
        int min_rate = p->min;
        int max_rate = p->max;
        struct snd_interval t;
        int err;

        err = query_hw_param(PARM_RATE, &min_rate, &max_rate);
        if (err < 0)
                return err;

        t.min = min_rate;
        t.max = max_rate;
        t.openmin = t.openmax = 0;
        t.integer = 1;

        return snd_interval_refine(p, &t);
}

The above is simplified not to allow the open min/max and assume only
integer, which should be enough for your cases, I suppose.

And the above function can be generalized like

static int hw_rule_interval(struct snd_pcm_hw_params *params,
                            struct snd_pcm_hw_rule *rule)
{
        struct snd_interval *p =
                hw_param_interval(params, rule->var);
        int min_val = p->min;
        int max_val = p->max;
        struct snd_interval t;
        int err;

        err = query_hw_param(alsa_parm_to_xen_parm(rule->var),
                        &min_val, &max_val);
        if (err < 0)
                return err;

        t.min = min_val;
        t.max = max_val;
        t.openmin = t.openmax = 0;
        t.integer = 1;

        return snd_interval_refine(p, &t);
}

and registering this via

        err = snd_pcm_hw_rule_add(runtime, 0, SNDRV_PCM_HW_PARAM_RATE,
                                  hw_rule_interval, NULL, -1);

In the above NULL can be referred in the callback via rule->private,
if you need some closure in the function, too.


Takashi

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