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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Jul 29, 2022 at 11:46:32AM -0500, Pierre-Louis Bossart wrote:
> 
> >>>>> 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?
> 
> I don't see what adding another layer of hierarchy would bring. In their
> simplest configuration, there are 6 registers 8-bit exposed. And the
> port registers, when present, are accessed with a plain vanilla offset.

Who uses these registers?

> > What uses these sysfs files today that would be confused about an empty
> > directory?
> 
> That's a good question. I am not aware of any tools making use of those
> attributes. To a large degree, they are helpful only for debug and
> support, all these read-only attributes could be moved to debugfs. That
> could be a way to simplify everyone's life....

That would be much nicer, put it all in a single debugfs file and it
would be so simple.

What attributes could we do that for?

thanks,

greg k-h