[PATCH v3 1/1] ASoC: fsl_asrc: make sure the input and output clocks are different

[Shengjiu Wang]

On Fri, Jul 17, 2020 at 6:40 PM Arnaud Ferraris
<arnaud.ferraris at collabora.com> wrote:
>
> The current clock selection algorithm might select the same clock for
> both input and output when, for instance, the output sample rate is a
> multiple of the input rate.
>
> In that case, both selectable clocks will be multiples of both the input
> and output sample rates, and therefore the first of these clocks will be
> selected for both input and output, leading to miscalculation of the
> dividers for either the input or output side.
>
> Example:
>   Input uses clock A (512kHz) and has a sample rate of 8kHz
>   Output uses clock B (1024kHz) and has a sample rate of 16kHz
>
>   In this case, the algorithm will select clock A for both input and
>   output: the input divider will therefore be calculated properly
>   (512 / 8 => 64), but the output divider's value will be only half
>   the expected value (512 / 16 => 32 instead of 1024 / 16 => 64).
>
(input divider, output divider) = (64, 32) for the same clock source
(512kHz) looks no problem.  could you explain more detail why
(64, 32) can't work?


> This patch makes sure it always selects distinct input and output
> clocks.

There should be no such constraint for this IP, do you have any
evidence for we should use distinct input and output clocks?

>
> Signed-off-by: Arnaud Ferraris <arnaud.ferraris at collabora.com>
> ---
>  sound/soc/fsl/fsl_asrc.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/sound/soc/fsl/fsl_asrc.c b/sound/soc/fsl/fsl_asrc.c
> index 02c81d2e34ad..de10c208d3c8 100644
> --- a/sound/soc/fsl/fsl_asrc.c
> +++ b/sound/soc/fsl/fsl_asrc.c
> @@ -608,8 +608,8 @@ static void fsl_asrc_select_clk(struct fsl_asrc_priv *asrc_priv,
>  {
>         struct fsl_asrc_pair_priv *pair_priv = pair->private;
>         struct asrc_config *config = pair_priv->config;
> -       int rate[2], select_clk[2]; /* Array size 2 means IN and OUT */
> -       int clk_rate, clk_index;
> +       int rate[2], select_clk[2], clk_index[2]; /* Array size 2 means IN and OUT */
> +       int clk_rate;
>         int i = 0, j = 0;
>
>         rate[IN] = in_rate;
> @@ -618,11 +618,15 @@ static void fsl_asrc_select_clk(struct fsl_asrc_priv *asrc_priv,
>         /* Select proper clock source for internal ratio mode */
>         for (j = 0; j < 2; j++) {
>                 for (i = 0; i < ASRC_CLK_MAP_LEN; i++) {
> -                       clk_index = asrc_priv->clk_map[j][i];
> -                       clk_rate = clk_get_rate(asrc_priv->asrck_clk[clk_index]);
> -                       /* Only match a perfect clock source with no remainder */
> +                       clk_index[j] = asrc_priv->clk_map[j][i];
> +                       clk_rate = clk_get_rate(asrc_priv->asrck_clk[clk_index[j]]);
> +                       /*
> +                        * Only match a perfect clock source with no remainder
> +                        * and make sure the input & output clocks are different
> +                        */
>                         if (clk_rate != 0 && (clk_rate / rate[j]) <= 1024 &&
> -                           (clk_rate % rate[j]) == 0)
> +                           (clk_rate % rate[j]) == 0 &&
> +                           (j == 0 || clk_index[j] != clk_index[j - 1]))
>                                 break;
>                 }
>
> --
> 2.27.0
>