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

From: Pierre-Louis Bossart
Date: Tue Jan 28 2020 - 11:02:37 EST



On 1/28/20 4:50 AM, Vinod Koul wrote:
Hi Pierre,

I took some time to look into the overall code and how this is fitting
into big picture and help recommend way forward.

Thanks Vinod, I appreciate the time you've taken to review this code.
This is constructive feedback.

On 21-01-20, 11:31, Pierre-Louis Bossart wrote:


A rename away from probe will certainly be very helpful as
you would also agree that terms 'probe' and 'remove' have a very
special meaning in kernel, so let us avoid these

ok, so would the following be ok with you?

/**
* struct sdw_md_driver - SoundWire 'Master Device' driver
*
* @init: allocations and initializations (hardware may not be enabled yet)
* @startup: initialization handled after the hardware is enabled, all
* clock/power dependencies are available
* @shutdown: cleanups before hardware is disabled (optional)
* @exit: free all remaining resources
* @autonomous_clock_stop_enable: enable/disable driver control while
* in clock-stop mode, typically in always-on/D0ix modes. When the driver
* yields control, another entity in the system (typically firmware
* running on an always-on microprocessor) is responsible to tracking
* Slave-initiated wakes
*/
struct sdw_md_driver {
int (*init)(struct sdw_master_device *md, void *link_ctx);
int (*startup)(struct sdw_master_device *md);
int (*shutdown)(struct sdw_master_device *md);
int (*exit)(struct sdw_master_device *md);
int (*autonomous_clock_stop_enable)(struct sdw_master_device *md,
bool state);
};

So this is a soundwire core driver structure, but the modelling and
explanation provided here suggests the reasoning to be based on hardware
sequencing. I am not sure if we should follow this approach. Solving
hardware sequencing is fine but that should IMO be restricted to intel
code as that is intel issue which may or may not be present on other
controllers.

I agree the directions are Intel-specific, there was no intention of requiring anyone to implement all these callbacks.

If I look at the calling sequence of the code (looked up the sof code on
github, topic/sof-dev-rebase), the sof code sound/soc/sof/intel/hda.c
invokes the sdw_intel_startup() and sdw_intel_probe() based on hardware
sequencing and further you call .init and .probe/startup of sdw_md_driver.

I really do not see why we need a sdw_md_driver object to do that. You can
easily have a another function exported by sdw_intel driver and you call
these and do same functionality without having a sdw_md_driver in
between.

I must admit I am beyond my comfort zone: I introduced this sdw_master_driver only based on Greg's recommendation to look at GreyBus.

All I care about are the three steps of
a. creating a device (needed because the ACPI description does not model the master hardware)
b. doing the required initializations and allocations
c. exposing a startup callback, and provide room for expansion for future evolutions.

If there's a better way to do this I don't mind at all. The value-added of these patches is not in how the device/driver model is used but rather the hardware sequencing and power management.

That said, I have two key points that I should explain further.

When we started this work, we initially did not have a notion of 'master driver' coupled with the 'master_device'. But we faced a number of issues with ASoC registrations, which seemed to required a driver to be registered and associated with the device. We kept this part as a separate commit to make sure this requirement wasn't lost:

https://github.com/thesofproject/linux/commit/28b934ce9c165e095dac5cf71da5685768831337

Looking back at my notes, this came from this piece of code:

static char *fmt_single_name(struct device *dev, int *id)
{
char *found, name[NAME_SIZE];
int id1, id2;

if (dev_name(dev) == NULL)
return NULL;

strlcpy(name, dev_name(dev), NAME_SIZE);

/* are we a "%s.%d" name (platform and SPI components) */
found = strstr(name, dev->driver->name); <<< oops

so for ASoC, there is an expectation that each device does have a driver associated with it. Just for fun I reverted the commit above and that immediately trigger an kernel oops.

The other point that we came across is that PM support is only enabled thanks to a hook in the driver structure:

static const struct dev_pm_ops intel_pm = {
SET_SYSTEM_SLEEP_PM_OPS(intel_suspend, intel_resume)
SET_RUNTIME_PM_OPS(intel_suspend_runtime, intel_resume_runtime, NULL)
};

struct sdw_master_driver intel_sdw_driver = {
.driver = {
.name = "intel-sdw",
.owner = THIS_MODULE,
.bus = &sdw_bus_type,
.pm = &intel_pm, <<<<
},
.probe = intel_master_probe,
.startup = intel_master_startup,
.process_wake_event = intel_master_process_wakeen_event,
.remove = intel_master_remove,
};

I am not aware of any other way to do this.

In short, I have two technical reasons we came across that made us rely on this 'master driver' object. I agree that the solution I suggested may not be very elegant or can be seen as overkill, but it does provide support for two integration requirements.

Now, I am going to step back one more step and ask you why should we
have a sdw_md_driver? I am not seeing the driver object achieving
anything here expect adding wrappers which we can avoid. But we still
need to add the sdw_master_device() as a new device object and use that
for both sysfs representation as well as representing the master device
and do all the things we want, but it *can* come without having
accompanying sdw_md_driver.

This way you can retain you calling sequence and add the master device.

Stretching this one more step I would ask that maybe it is even better
idea that we should hide sdw_master_device_add() calling for soundwire
drivers and move that internal to bus as part of bus registration as
well, I don't see sdw_master_device calling back into the driver so it
should not impact your sequences as well.

Do you see a reason for sdw_md_driver which is must have? I couldn't
find that by looking at the code, let me know if I have missed anything
here.

Yes, the two reasons explained above.

That said, one possible compromize would be to remove those callbacks and indirections, such md->driver->remove(md);

we could perfectly export something like

intel_device_remove(md)

which would directly call the relevant functions. that would essentially remove the notion of 'sdw_master_driver' but keep the bare minimal driver structure that's needed.

Note that the removal of platform devices is not something I wanted to enforce across the board. If Qualcomm are happy with the platform devices, that's fine with me. I only want to work around the two limitations I mentioned (ACPI does not support masters, only controllers, and power rail dependencies).

So to summarize, my recommendation would be to drop sdw_md_driver, keep
sdw_master_device object and make sdw_master_device_add() hidden to
driver and call it from sdw_add_bus_master() and keep intel specific
startup/init routine which do same steps as they have now.

I am afraid the sequence you suggest is not technically possible: sdw_add_bus_master() requires a device to be created already, see e.g. how it's used by Qualcomm:

https://github.com/thesofproject/linux/blob/4026efd12ac983d363fb43c37a8af741a2c90dc8/drivers/soundwire/qcom.c#L811

sdw_add_bus_master() is called from the platform device probe.
I cannot insert a device creation in this code.

It'd actually be counter productive to add device management at the bus level, since we'd soon have incompatibilities between Intel, Qualcomm and others. What is platform-specific should be handled outside of the bus layer.

I hope this email clarifies some of my points?

I'll try to nooddle with the notion of bare minimum driver, that looks like a possible alternative that solves my problems and would address your concerns.

Thanks
-Pierre