Re: [RFC PATCH 1/5] drivers : introduce device_path api

[Andy Green <andy.green-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On 11/28/2012 02:09 AM, the mail apparently from Alan Stern included:
> On Wed, 28 Nov 2012, Andy Green wrote:
>
>>> Greg's advice was simply not to rely on pathnames in sysfs because they
>>> aren't fixed in stone.  That leaves plenty of other ways to approach
>>> this problem.
>>
>> It's sage advice, but there is zero code provided in my patches that
>> "relies on pathnames in sysfs".
>
> In your 1/5 patch, _device_path_generate() concatenates device name
> strings, starting from a device root and separating elements with '/'
> characters.  Isn't that the same as a sysfs pathname?

It's nothing to do with sysfs... yes some unrelated bits of sysfs also 
walk the device path.  If we want to talk about how fragile the device 
path is as an id scheme over time we need to talk about likelihood of 
individual device names changing, not "sysfs".  Anyway -->

>>> Basically, what you want is for something related to device A (the
>>> regulator or the GPIO) to happen whenever device B (the ehci-omap.0
>>> platform device) is bound to a driver.  The most straightforward way to
>>> arrange this is for A's driver to have a callback that is invoked
>>> whenever B is bound or unbound.  The most straightforward way to
>>> arrange _that_ is to allow each platform_device to have a list of
>>> callbacks.
>>
>> Sorry I didn't really understand this proposal yet.  You want "A", the
>> regulator, driver to grow a callback function that gets called when the
>> targeted platform_device ("B", ehci-omap.0) probe happens.  Could you
>> expand what the callback prototype or new members in the struct might
>> look like?  It's your tuple thing or we pass it an opaque pointer that
>> is the struct regulator * or suchlike?
>
> Well, it won't be exactly the same as the tuple thing because no
> strings will be involved, but it would be similar.  The callback would
> receive an opaque pointer (presumably to the regulator) and a device
> pointer (the B device).

OK.  So I try to sketch it out iteractively to try to get in sync:

device.h:

	enum asset_event {
		AE_PROBED,
		AE_REMOVED
	};

	struct device_asset {
		char *name; /* name of regulator, clock, etc */
		void *asset; /* regulator, clock, etc */
		int (*handler)(struct device *dev_owner, enum asset_event asset_event, 
struct device_asset *asset);
	};

	struct device {
	...
		struct device_asset *assets;
	...
	};


drivers/base/dd.c | really_probe():

...
	struct device_asset *asset;
...
	asset = dev->assets;
	while (asset && asset->name) {
		if (asset->handler(dev, AE_PROBED, asset)) {
			/* clean up and bail */
		}
		asset++;
	}

	/* do probe */
...


drivers/base/dd.c | __device_release_driver:  (is this really the best 
place to oppose probe()?)

...
	struct device_asset *asset;
...

	/* call device ->remove() */
...
	asset = dev->assets;
	while (asset && asset->name) {
		asset->handler(dev, AE_REMOVED, asset);
		asset++;
	}
...


board file:

	static struct regulator myreg = {
		.name = "mydevice-regulator",
	};

	static struct device_asset mydevice_assets[] = {
		{
			.name = "mydevice-regulator",
			.handler = regulator_default_asset_handler,
		},
		{ }
	};

	static struct platform_device mydevice = {
	...
		.dev = {
			.assets = mydevice_assets,
		},
	...
	};


regulator code:

int regulator_default_asset_handler(struct device *dev_owner, enum 
asset_event asset_event, struct device_asset *asset)
{
	struct regulator **reg = &asset->asset;
	int n;

	switch (asset_event) {
	case AE_PROBED:
		*reg = regulator_get(dev_owner, asset->name);
		if (IS_ERR(*reg))
			return *reg;
		n = regulator_enable(*reg);
		if (n < 0)
			regulator_put(*reg);
		return n;

	case AE_REMOVED:
		regulator_put(*reg);
		break;
	}

	return 0;
}
EXPORT_SYMBOL_GPL(regulator_default_asset_handler);


The subsystems that can expect to get used (clock...) might each want to 
define a default handler like the one for regulator.  That'll be an end 
to the code duplication issue.  The user can still do his own handler if 
he wants.

I put a name field in so we can use regulator_get() nicely, we don't 
need access to the object pointer or that it exists at boardfile-time 
that way either.  But I can see it's arguable.

>> Throwing out the path stuff and limiting this to platform_device means
>> you cannot bind to dynamically created objects like hub or anything
>> downstream of a hub.  So Felipe's identification of the hub as the
>> happening place to do this is out of luck.
>
> Greg pointed out that this could be useful for arbitrary devices, not
> just platform devices, so it could be applied to dynamically created
> objects.

Well that is cool, but to exploit that in the dynamic object case 
arrangements for identifying the appropriate object has appeared are 
needed.  We have a nice array of platform_devices nicely there in the 
board file we can attach assets to like "pdev[1].dev.assets = xxx;" but 
that's not there in dynamic device case.  Anyway this sounds like what 
we're discussing can be well worth establishing and might lead to that 
later.

> As for what Felipe said...  He suggested doing this when the hub driver
> binds to the controller's root hub.  The root hub is created when the
> controller's driver registers the new USB bus.  This happens as part of
> the driver's probe routine.  So what I have been talking about is very
> similar (in terms of when it happens) to what Felipe wanted.
>
> Besides, Felipe wasn't thinking in the most general terms.  (In fact,
> at first he confused the root hub with the LAN95xx's hub.)  There's no
> reason to restrict this sort of thing to USB hubs (or to regulators,
> for that matter).  The driver core is the right place for it.

Sounds good to me.

-Andy

-- 
Andy Green | TI Landing Team Leader
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