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

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Date:   Fri, 29 Jul 2022 11:46:32 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Vinod Koul <vkoul@...nel.org>, alsa-devel@...a-project.org,
        Bard Liao <yung-chuan.liao@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Sanyog Kale <sanyog.r.kale@...el.com>
Subject: Re: [PATCH 2/5] soundwire: sysfs: cleanup the logic for creating the
 dp0 sysfs attributes


>>>>> 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.

> 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....