linux-kernel - Re: [alsa-devel] [PATCH v5 09/17] soundwire: intel: remove platform devices and use 'Master Devices' instead

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Date:   Mon, 6 Jan 2020 11:12:21 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
Cc:     alsa-devel@...a-project.org, tiwai@...e.de,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        broonie@...nel.org, srinivas.kandagatla@...aro.org,
        jank@...ence.com, slawomir.blauciak@...el.com,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Bard liao <yung-chuan.liao@...ux.intel.com>,
        Rander Wang <rander.wang@...ux.intel.com>
Subject: Re: [alsa-devel] [PATCH v5 09/17] soundwire: intel: remove platform
 devices and use 'Master Devices' instead

On 27-12-19, 18:13, Pierre-Louis Bossart wrote:
> 
> 
> > > +extern struct sdw_md_driver intel_sdw_driver;
> > 
> > who uses this intel_sdw_driver? I would assumed someone would register
> > this with the core...
> 
> this is a structure used by intel_init(), see the following code.
> 
> +		md = sdw_md_add(&intel_sdw_driver,
> +				res->parent,
> +				acpi_fwnode_handle(adev),
> +				i);
> 
> that will in turn call intel_master_probe() as defined below:
> 
> +struct sdw_md_driver intel_sdw_driver = {
> +	.probe = intel_master_probe,
> +	.startup = intel_master_startup,
> +	
> 
> > > -		link->pdev = pdev;
> > > -		link++;
> > > +		/* let the SoundWire master driver to its probe */
> > > +		md->driver->probe(md, link);
> > 
> > So you are invoking driver probe here.. That is typically role of driver
> > core to do that.. If we need that, make driver core do that for you!
> > 
> > That reminds me I am missing match code for master driver...
> 
> There is no match for the master because it doesn't have an existence in
> ACPI. There are no _ADR or HID that can be used, the only thing that exists
> is the Controller which has 4 sublinks. Each master must be added  by hand.
> 
> Also the SoundWire master cannot be enumerated or matched against a
> SoundWire bus, since it controls the bus itself (that would be a chicken and
> egg problem). The SoundWire master would need to be matched on a parent bus
> (which does not exist for Intel) since the hardware is embedded in a larger
> audio cluster that's visible on PCI only.
> 
> Currently for Intel platforms, the SoundWire master device is created by the
> SOF driver (via the abstraction in intel_init.c).

That is okay for me, the thing that is bit confusing is having a probe
etc and no match.. (more below)..

> > So we seem to be somewhere is middle wrt driver probing here! IIUC this
> > is not a full master driver, thats okay, but then it is not
> > completely transparent either...
> > 
> > I was somehow thinking that the driver will continue to be
> > 'platform/acpi/of' driver and master device abstraction will be
> > handled in the core (for example see how the busses like i2c handle
> > this). The master device is created and used to represent but driver
> > probing etc is not done
> 
> I2C controllers are typically PCI devices or have some sort of ACPI
> description. This is not the case for SoundWire masters on Intel platforms,

Well the world is not PCI/ACPI... We have controllers which are DT
described and work in same manner as a PCI device.

> so even if I wanted to I would have no ability to implement any matching or
> parent bus registration.
> 
> Also the notion of 'probe' does not necessarily mean that the device is
> attached to a bus, we use DAI 'drivers' in ASoC and still have probe/remove
> callbacks.

The "big" difference is that probe is called by core (asoc) and not by
driver onto themselves.. IMO that needs to go away.

> And if you look at the definitions, we added additional callbacks since
> probe/remove are not enough to deal with hardware restrictions:
> 
> For Intel platforms, we have a startup() callback which is only invoked once
> the DSP is powered and the rails stable. Likewise we added an
> 'autonomous_clock_stop()' callback which will be needed when the Linux
> driver hands-over control of the hardware to the DSP firmware, e.g. to deal
> with in-band wakes in D0i3.
> 
> FWIW, the implementation here follows what was suggested for Greybus 'Host
> Devices' [1] [2], so it's not like I am creating any sort of dangerous
> precedent.
> 
> [1]
> https://elixir.bootlin.com/linux/latest/source/drivers/greybus/es2.c#L1275
> [2] https://elixir.bootlin.com/linux/latest/source/drivers/greybus/hd.c#L124

And if you look closely all this work is done by core not by drivers!
Drivers _should_ never do all this, it is the job of core to do that for
you.

-- 
~Vinod