[PATCH] ALSA: core: sysfs: show components string

[Pierre-Louis Bossart]

On 3/23/20 8:53 PM, Takashi Sakamoto wrote:
> Hi,
> 
> On Mon, Mar 23, 2020 at 02:36:23PM -0500, Pierre-Louis Bossart wrote:
>> Add attribute and show the components string. This is useful to see
>> what is provided to userspace and e.g. used by UCM to understand the
>> card configuration:
>>
>> root at plb:~# more /sys/class/sound/card0/components
>> HDA:8086280b,80860101,00100000 cfg-spk:2 hs:rt711 spk:rt1308 mic:rt715
>>
>> Note that the style uses what's recommended by checkpatch.pl and is
>> slightly different from other sysfs attributes.
>>
>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart at linux.intel.com>
>> ---
>>   sound/core/init.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>   
> I have a concern about this patch in a point of userspace interface.
> 
> The 'component' field of 'struct snd_ctl_card_info' is just defined to
> have strings with space-separated chunks, and its actual value is not
> so fine-documented, thus it's largely different depending of developers
> of each driver.

In case you missed it, the components are used by machine drivers to 
report e.g. number of speakers, mics, etc, so that UCM can find the 
right configuration. For a given family of products, the syntax will be 
fixed, e.g. hs stands for headset codec, etc.

> $ cat include/uapi/sound/asound.h
>       ...
>   941 struct snd_ctl_card_info {
>           ...
>   950     unsigned char components[128];  /* card components / fine identification, delimited with one space (AC97 etc..) */
>   951 };
> 
> On the other hand, the node of sysfs is quite common in Linux because
> it's tightly coupled to kernel objects. Let you see files under
> 'Documentation/ABI/'. We can find efforts to maintain sysfs node so
> safe and stable. Due to this reason, it's better to take more care when
> adding new node.
> 
> Would I request you the reason to add this node and the reason that
> current ALSA control interface doesn't satisfy your requirement?

simplicity for user support. Anyone can peak at a sysfs file, not 
everyone is familiar with the alsa control interface.

We get lots of bug reports from people who are asking for configuration 
help, and the simpler the command is the better.

>> diff --git a/sound/core/init.c b/sound/core/init.c
>> index b02a99766351..decaf944a8ad 100644
>> --- a/sound/core/init.c
>> +++ b/sound/core/init.c
>> @@ -695,9 +695,21 @@ card_number_show_attr(struct device *dev,
>>   
>>   static DEVICE_ATTR(number, 0444, card_number_show_attr, NULL);
>>   
>> +static ssize_t
>> +components_show(struct device *dev,
>> +		struct device_attribute *attr, char *buf)
>> +{
>> +	struct snd_card *card = container_of(dev, struct snd_card, card_dev);
>> +
>> +	return scnprintf(buf, PAGE_SIZE, "%s\n", card->components);
>> +}
>> +
>> +static DEVICE_ATTR_RO(components);
>> +
>>   static struct attribute *card_dev_attrs[] = {
>>   	&dev_attr_id.attr,
>>   	&dev_attr_number.attr,
>> +	&dev_attr_components.attr,
>>   	NULL
>>   };
>>   
>> -- 
>> 2.20.1
>   
> 
> Regards
> 
> Takashi Sakamoto
>