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

[Pierre-Louis Bossart]

On 7/29/22 12:15, Greg Kroah-Hartman wrote:
> 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?

The bus layer. When a 'stream' is created, the 'bit allocation' will
define who owns which bitSlots in the frame and the registers will be
programmed. The bit allocation may be dynamic or fixed depending on the
host.

>>> 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?

All of them really - except maybe the device number which could be used
to figure what the device is when looking at power status and other
'standard' sysfs attributes. sdw:3:025d:0714:01 is not really
user-friendly, device_number 1 is.