Re: [PATCH 2/5] soundwire: sysfs: cleanup the logic for creating the dp0 sysfs attributes

[Greg Kroah-Hartman]

On Fri, Jul 29, 2022 at 10:52:28AM -0500, Pierre-Louis Bossart wrote:
> 
> >>>>> diff --git a/drivers/soundwire/sysfs_slave.c b/drivers/soundwire/sysfs_slave.c
> >>>>> index 83e3f6cc3250..3723333a5c2b 100644
> >>>>> --- a/drivers/soundwire/sysfs_slave.c
> >>>>> +++ b/drivers/soundwire/sysfs_slave.c
> >>>>> @@ -174,6 +174,16 @@ static ssize_t words_show(struct device *dev,
> >>>>>  }
> >>>>>  static DEVICE_ATTR_RO(words);
> >>>>>  
> >>>>> +static umode_t dp0_is_visible(struct kobject *kobj, struct attribute *attr,
> >>>>> +			      int n)
> >>>>> +{
> >>>>> +	struct sdw_slave *slave = dev_to_sdw_dev(kobj_to_dev(kobj));
> >>>>> +
> >>>>> +	if (slave->prop.dp0_prop)
> >>>>> +		return attr->mode;
> >>>>> +	return 0;
> >>>>> +}
> >>>>
> >>>> This changes the results slightly by creating an empty 'dp0' directory
> >>>> with no attributes inside.
> >>>>
> >>>> Before:
> >>>>
> >>>> [root@fedora ~]# cd /sys/bus/soundwire/devices/sdw:3:025d:0714:01
> >>>> [root@fedora sdw:3:025d:0714:01]# ls dp0
> >>>> ls: cannot access 'dp0': No such file or directory
> >>>>
> >>>> After:
> >>>> [root@fedora sdw:3:025d:0714:01]# ls dp0
> >>>
> >>> That should be fine, tools should just be looking for the attributes,
> >>> not the existance of a directory, right?
> >>
> >> The idea what that we would only expose ports that actually exist.
> >> That's helpful information anyone with a basic knowledge of the
> >> SoundWire specification would understand.
> > 
> > Is "dp0" a port?  If so, why isn't it a real device?
> 
> The SoundWire spec defines the concept of 'data port'. The valid ranges
> are 1..14, but in all existing devices the number of data ports is way
> smaller, typically 2 to 4. Data ports (DPn) are source or sink, and
> there's no firm rule that data ports needs to be contiguous.
> 
> DP0 is a 'special case' where the data transport is used for control
> information, e.g. programming large set of registers or firmware
> download. DP0 is completely optional in hardware, and not handled in
> Linux for now.
> 
> DP0 and DPn expose low-level transport registers, which define how the
> contents of a FIFO will be written or read from the bus. Think of it as
> a generalization of the concept of TDM slots, where instead of having a
> fixed slot per frame the slot position/repetition/runlength can be
> programmed.
> 
> The data ports could be as simple as 1-bit PDM, or support 8ch PCM
> 24-bits. That's the sort of information reported in attributes.

Why not make them a real device like we do for USB endpoints?

What uses these sysfs files today that would be confused about an empty
directory?

thanks,

greg k-h