linux-kernel - Re: [alsa-devel] [PATCH v5 10/15] soundwire: Add sysfs for SoundWire DisCo properties

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Date:   Wed, 13 Dec 2017 22:22:46 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     ALSA <alsa-devel@...a-project.org>,
        Charles Keepax <ckeepax@...nsource.cirrus.com>,
        Sudheer Papothi <spapothi@...eaurora.org>,
        Takashi <tiwai@...e.de>, plai@...eaurora.org,
        LKML <linux-kernel@...r.kernel.org>,
        Pierre <pierre-louis.bossart@...ux.intel.com>,
        patches.audio@...el.com, Mark <broonie@...nel.org>,
        srinivas.kandagatla@...aro.org,
        Sagar Dharia <sdharia@...eaurora.org>, alan@...ux.intel.com
Subject: Re: [alsa-devel] [PATCH v5 10/15] soundwire: Add sysfs for SoundWire
 DisCo properties

On Wed, Dec 13, 2017 at 05:28:21PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Dec 13, 2017 at 03:24:30PM +0530, Vinod Koul wrote:
> > On Wed, Dec 13, 2017 at 10:15:37AM +0100, Greg Kroah-Hartman wrote:
> > > On Wed, Dec 06, 2017 at 09:17:06PM +0530, Vinod Koul wrote:
> > > > It helps to read the properties for understanding and debugging
> > > > systems, so add sysfs files for SoundWire DisCo properties.
> > > > 
> > > > TODO: Add ABI files for sysfs
> > > 
> > > Is this TODO done?
> > 
> > Nope sorry not yet. But before this get merged will add
> > 
> > > > + * Base file is:
> > > > + *		properties
> > > > + *		|---- interface-revision
> > > > + *		|---- master-count
> > > > + *		|---- link-N
> > > > + *		      |---- clock-stop-modes
> > > > + *		      |---- max-clock-frequency
> > > > + *		      |---- clock-frequencies
> > > > + *		      |---- default-frame-rows
> > > > + *		      |---- default-frame-cols
> > > > + *		      |---- dynamic-frame-shape
> > > > + *		      |---- command-error-threshold
> > > > + */
> > > 
> > > Why nest them so deep?  Anyway, that's not really an issue I guess, it's
> > > your ABI, not mine :)
> > 
> > well it gives us a hierarchical view. We have N links...
> 
> That's fine, then make it a real 'struct device' if you want to have a
> reference counted object.  Tie it to your bus, and you are good to go.
> Don't use a raw kobject as that totaly breaks the device heirachy in the
> kernel as well as preventing any of these attributes from being accessed
> by userspace libraries (i.e. libudev.)
> 
> > > > +
> > > > +struct sdw_master_sysfs {
> > > > +	struct kobject kobj;
> > > > +	struct sdw_bus *bus;
> > > 
> > > Huh?  Why do you need to use kobjects?
> > > 
> > > When you switch from using a 'struct device' and hang a kobject off of
> > > it, that's a huge signal that something is wrong here.  That kobject
> > > will now no longer be part of the device "chain" in the system, uevents
> > > will get odd, and other strange things can happen.
> > > 
> > > Why can't you just use "normal" attributes attached to the device?  You
> > > shouldn't need a kobject here.  What am I missing?
> > 
> > Okay my understanding might be incorrect then. So we can have N links in
> > the system and each link would have a kobject for "link-N". Not sure how
> > device attributes would do link-N/clock-stop-modes and so on, if they can
> > let me know how and I will surely change that.
> 
> You can create a subdirectory for attributes quite easily.  If you don't
> want to make it a "full" object, and all you care about is the
> subdirectory, then do it that way.  Otherwise use a 'struct device'
> please.

Okay thanks this makes sense, yes all we do care about is creating
subdirectories and attributes under them. So in that sense we don't care
much about adding kobjects, it was means to the end.

So do you mind pointing an example, I though the way for that was kobjects
by looking at few examples I saw.

-- 
~Vinod