[alsa-devel] ASoC: davinci-mcasp: Set rule constraints if implicit BCLK divider is used

[Jyri Sarha]

On 04/10/15 12:33, Dan Carpenter wrote:
> Hello Jyri Sarha,
>
> The patch a75a053f1eef: "ASoC: davinci-mcasp: Set rule constraints if
> implicit BCLK divider is used" from Mar 20, 2015, leads to the
> following Sparse warning:
>
> 	sound/soc/davinci/davinci-mcasp.c:1098:45:
> 	warning: Variable length array is used.
>
> sound/soc/davinci/davinci-mcasp.c
>    1088  static int davinci_mcasp_hw_rule_channels(struct snd_pcm_hw_params *params,
>    1089                                            struct snd_pcm_hw_rule *rule)
>    1090  {
>    1091          struct davinci_mcasp_ruledata *rd = rule->private;
>    1092          struct snd_interval *ci =
>    1093                  hw_param_interval(params, SNDRV_PCM_HW_PARAM_CHANNELS);
>    1094          int sbits = params_width(params);
>    1095          int rate = params_rate(params);
>    1096          int max_chan_per_wire = rd->mcasp->tdm_slots < ci->max ?
>    1097                  rd->mcasp->tdm_slots : ci->max;
>    1098          unsigned int list[ci->max - ci->min + 1];
>                                    ^^^^^^^^^^^^^^^^^^^^^
> I wasnt able to determine where these values are capped.  The worry is
> that the kernel only has an 8k stack so if they are too high it could
> oops.  I think these values can come from the user in
> snd_pcm_hw_params_user() but I'm an newbie to the code and I didn't
> see where the limits were enforced.
>

In theory the ci->max should never be more than 512, because
static struct snd_soc_dai_driver davinci_mcasp_dai[] = {
...
			.channels_max	= 32 * 16,

should limit the maximum allowed channel index. Normally this value 
should be way smaller.

However, since there is considerable amount of code involved it is hard 
to be produce a proof of that. I could add a hard max limit of 512 - or 
even 128 - to the list size. The cases when the list size would need to 
be even close 128 are quite theoretical.

Best regards,
Jyri

>    1099          int c1, c, count = 0;
>    1100
>    1101          for (c1 = ci->min; c1 <= max_chan_per_wire; c1++) {
>    1102                  uint bclk_freq = c1*sbits*rate;
>    1103                  int ppm;
>
>
> regards,
> dan carpenter
>