Re: [alsa-devel] [RFC PATCH 1/7] soundwire: Add sysfs support for master(s)

Vinod Koul
2019-05-09T04:26:45Z

On 08-05-19, 15:57, Pierre-Louis Bossart wrote:
>
>
> On 5/8/19 11:59 AM, Greg KH wrote:
> > On Wed, May 08, 2019 at 11:42:15AM -0500, Pierre-Louis Bossart wrote:
> > >
> > >
> > > On 5/8/19 4:16 AM, Greg KH wrote:
> > > > On Wed, May 08, 2019 at 01:16:06PM +0530, Vinod Koul wrote:
> > > > > On 07-05-19, 17:49, Pierre-Louis Bossart wrote:
> > > > > >
> > > > > > > > The model here is that Master device is PCI or Platform device and then
> > > > > > > > creates a bus instance which has soundwire slave devices.
> > > > > > > >
> > > > > > > > So for any attribute on Master device (which has properties as well and
> > > > > > > > representation in sysfs), device specfic struct (PCI/platfrom doesn't
> > > > > > > > help). For slave that is not a problem as sdw_slave structure takes care
> > > > > > > > if that.
> > > > > > > >
> > > > > > > > So, the solution was to create the psedo sdw_master device for the
> > > > > > > > representation and have device-specific structure.
> > > > > > >
> > > > > > > Ok, much like the "USB host controller" type device. That's fine, make
> > > > > > > such a device, add it to your bus, and set the type correctly. And keep
> > > > > > > a pointer to that structure in your device-specific structure if you
> > > > > > > really need to get to anything in it.
> > > > > >
> > > > > > humm, you lost me on the last sentence. Did you mean using
> > > > > > set_drv/platform_data during the init and retrieving the bus information
> > > > > > with get_drv/platform_data as needed later? Or something else I badly need
> > > > > > to learn?
> > > > >
> > > > > IIUC Greg meant we should represent a soundwire master device type and
> > > > > use that here. Just like we have soundwire slave device type. Something
> > > > > like:
> > > > >
> > > > > struct sdw_master {
> > > > > struct device dev;
> > > > > struct sdw_master_prop *prop;
> > > > > ...
> > > > > };
> > > > >
> > > > > In show function you get master from dev (container of) and then use
> > > > > that to access the master properties. So int.sdw.0 can be of this type.
> > > >
> > > > Yes, you need to represent the master device type if you are going to be
> > > > having an internal representation of it.
> > >
> > > Humm, confused...In the existing code bus and master are synonyms, see e.g.
> > > following code excerpts:
> > >
> > > * sdw_add_bus_master() - add a bus Master instance
> > > * @bus: bus instance
> > > *
> > > * Initializes the bus instance, read properties and create child
> > > * devices.
> > >
> > > struct sdw_bus {
> > > struct device *dev; <<< pointer here
> >
> > That's the pointer to what? The device that the bus is "attached to"
> > (i.e. parent, like a platform device or a pci device)?
> >
> > Why isn't this a "real" device in itself?

Correct, I am revisiting this and I think I have a fair idea of
expectations here (looking at usb and greybus model), will hack
something up

> Allow me to provide a bit of background. I am not trying to be pedantic but
> make sure we are on the same page.
>
> The SoundWire spec only defines a Master and Slaves attached to that Master.
>
> In real applications, there is a need to have multiple links, which can
> possibly operate in synchronized ways, so Intel came up with the concept of
> Controller, which expose multiple Master interfaces that are in sync (two
> streams can start at exactly the same clock edge of different links).
>
> The Controller is exposed in ACPI as a child of the HDAudio controller (ACPI
> companion of a PCI device). The controller exposes a 'master-count' and a
> set of link-specific properties needed for bandwidth/clock scaling.
>
> For some reason, our Windows friends did not want to have a device for each
> Master interface, likely because they did not want to load a driver per
> Master interface or have 'yellow bangs'.
>
> So the net result is that we have the following hierarchy in ACPI
>
> Device(HDAS) // HDaudio controller
> Device(SNDW) // SoundWire Controller
> Device(SDW0) { // Slave0
> _ADR(link0, vendorX, partY...)
> }
> Device(SDW1) { // Slave0
> _ADR(link0, vendorX, partY...)
> }
> Device(SDW2) { // Slave0
> _ADR(link1, vendorX, partY...)
> }
> Device(SDWM) { // Slave0
> _ADR(linkM, vendorX, partY...)
> }
>
> There is no master device represented in ACPI and the only way by which we
> know to which Master a Slave device is attached by looking up the _ADR which
> contains the link information.
>
> So, coming back to the plot, when we parse the Controller properties, we
> find out how many Master interfaces we have, create a platform_device for
> each of them, then initialize all the bus stuff.

So the idea here would be to go back and create a sdw_master device and
use that in the bus instance. I think it should be doable..

> > I thought I asked that a long time ago when first reviewing these
> > patches...

Sorry my fault, I should have fixed it back then.

> >
> > > unsigned int link_id;
> > > struct list_head slaves;
> > > DECLARE_BITMAP(assigned, SDW_MAX_DEVICES);
> > > struct mutex bus_lock;
> > > struct mutex msg_lock;
> > > const struct sdw_master_ops *ops;
> > > const struct sdw_master_port_ops *port_ops;
> > > struct sdw_bus_params params;
> > > struct sdw_master_prop prop;
> > >
> > > The existing code creates a platform_device in
> > > drivers/soundwire/intel_init.c, and it's assigned by the following code:
> >
> > The core creates a platform device, don't assume you can "take it over"
> > :)
> >
> > That platform device lives on the platform bus, you need a "master"
> > device that lives on your soundbus bus.
> >
> > Again, look at how USB does this. Or better yet, greybus, as that code
> > is a lot smaller and simpler.
>
> The learning curve is not small here...
>
> > >
> > > static int intel_probe(struct platform_device *pdev)
> > > {
> > > struct sdw_cdns_stream_config config;
> > > struct sdw_intel *sdw;
> > > int ret;
> > >
> > > sdw = devm_kzalloc(&pdev->dev, sizeof(*sdw), GFP_KERNEL);
> > > [snip]
> > > sdw->cdns.dev = &pdev->dev;
> > > sdw->cdns.bus.dev = &pdev->dev;
> >
> > Gotta love the lack of reference counting :(
> >
> > > I really don't see what you are hinting at, sorry, unless we are talking
> > > about major surgery in the code.

Not really we have object here which should contain a real device for
master and need plumbing for it..

> > It sounds like you need a device on your bus that represents the master,
> > as you have attributes associated with it, and other things. You can't
> > put attributes on a random pci or platform device, as you do not "own"
> > that device.
> >
> > does that help?
>
> Looks like we are doing things wrong at multiple levels.
>
> It might be better to have a more 'self-contained' solution where the bus
> initialization creates/registers a master device instead of having this
> proxy platform_device. That would avoid all these refcount issues and make
> the translation from device to bus straightforward.

yes that is my thinking as well. We still need to link to
platform/pci/whatever device you have and grab a refcount to that one.

> Am I on the right track or still in the weeds?

--
~Vinod