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 17:59:22 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Vinod Koul <vinod.koul@...el.com>
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 10:22:46PM +0530, Vinod Koul wrote:
> 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.

Create a dynamic attribute group on the fly and initialize it and set
the name to the name you want for the subdirectory.  I know it's done in
the kernel in some places, dig around :)

good luck!

greg k-h