Subject: Re: [RFC PATCH] platform: Faciliatate the creation of pseduo-platform busses

From: Patrick Pannuto <[email-masked]>
Date: Tue, 03 Aug 2010 17:02:29 -0700

On 08/03/2010 04:56 PM, Greg KH wrote:
> On Tue, Aug 03, 2010 at 04:35:06PM -0700, Patrick Pannuto wrote:
>> Inspiration for this comes from:
>> http://www.mail-archive.com/[email-masked]/msg31161.html
>>
>> INTRO
>>
>> As SOCs become more popular, the desire to quickly define a simple,
>> but functional, bus type with only a few unique properties becomes
>> desirable. As they become more complicated, the ability to nest these
>> simple busses and otherwise orchestrate them to match the actual
>> topology also becomes desirable.
>>
>> EXAMPLE USAGE
>>
>> /arch/ARCH/MY_ARCH/my_bus.c:
>>
>> 	#include <linux/device.h>
>> 	#include <linux/platform_device.h>
>>
>> 	struct bus_type my_bus_type = {
>> 		.name	= "mybus",
>> 	};
>> 	EXPORT_SYMBOL_GPL(my_bus_type);
>>
>> 	struct platform_device sub_bus1 = {
>> 		.name		= "sub_bus1",
>> 		.id		= -1,
>> 		.dev.bus	= &my_bus_type,
>> 	}
>> 	EXPORT_SYMBOL_GPL(sub_bus1);
> 
> You really want a bus hanging off of a bus?  Normally you need a device
> to do that, which is what I think you have here, but the naming is a bit
> odd to me.
> 
> What would you do with this "sub bus"?  It's just a device, but you are
> wanting it to be around for something.
> 

It's for power management stuff, basically, there are actual physical buses
involved that can be completely powered off IFF all of their devices are
not in use. Plus it actually matches bus topology this way.

>>
>> 	struct platform_device sub_bus2 = {
>> 		.name		= "sub_bus2",
>> 		.id		= -1,
>> 		.dev.bus	= &my_bus_type,
>> 	}
>> 	EXPORT_SYMBOL_GPL(sub_bus2);
>>
>> 	static int __init my_bus_init(void)
>> 	{
>> 		int error;
>> 		platform_bus_type_init(&my_bus_type);
>>
>> 		error = bus_register(&my_bus_type);
>> 		if (error)
>> 			return error;
>>
>> 		error = platform_device_register(&sub_bus1);
>> 		if (error)
>> 			goto fail_sub_bus1;
>>
>> 		error = platform_device_register(&sub_bus2);
>> 		if (error)
>> 			goto fail_sub_bus2;
>>
>> 		return error;
>>
>> 	fail_sub_bus2:
>> 		platform_device_unregister(&sub_bus1);
>> 	fail_sub_bus2:
>> 		bus_unregister(&my_bus_type);
>>
>> 		return error;
>> 	}
>> 	postcore_initcall(my_bus_init);
>> 	EXPORT_SYMBOL_CPY(my_bus_init);
>>
>> /drivers/my_driver.c
>> 	static struct platform_driver my_driver = {
>> 		.driver	= {
>> 			.name	= "my-driver",
>> 			.owner	= THIS_MODULE,
>> 			.bus	= &my_bus_type,
>> 		},
>> 	};
>>
>> /somewhere/my_device.c
>> 	static struct platform_device my_device = {
>> 		.name		= "my-device",
>> 		.id		= -1,
>> 		.dev.bus	= &my_bus_type,
>> 		.dev.parent	= &sub_bus_1.dev,
>> 	};
> 
> Ah, you put devices on this "sub bus".  But why?  Why not just put it on
> your "normal" bus that you created?  What's with the extra level of
> nesting here?
> 
> Other than that, it looks like a nice idea, are there portions of kernel
> code that can be simplified because of this?

See above.

> 
>> @@ -539,12 +541,12 @@ int __init_or_module platform_driver_probe(struct platform_driver *drv,
>>  	 * if the probe was successful, and make sure any forced probes of
>>  	 * new devices fail.
>>  	 */
>> -	spin_lock(&platform_bus_type.p->klist_drivers.k_lock);
>> +	spin_lock(&drv->driver.bus->p->klist_drivers.k_lock);
>>  	drv->probe = NULL;
>>  	if (code == 0 && list_empty(&drv->driver.p->klist_devices.k_list))
>>  		retval = -ENODEV;
>>  	drv->driver.probe = platform_drv_probe_fail;
>> -	spin_unlock(&platform_bus_type.p->klist_drivers.k_lock);
>> +	spin_unlock(&drv->driver.bus->p->klist_drivers.k_lock);
>>  
>>  	if (code != retval)
>>  		platform_driver_unregister(drv);
> 
> I'm guessing that this chunk can be applied now, right?

Probably; right now this will always work since anything that
is a platform_driver will have .driver.bus = &platform_bus_type,
but that does change with this patch.

There is no need (IMHO) for it to be a separate patch.

> 
>> @@ -1017,6 +1019,26 @@ struct bus_type platform_bus_type = {
>>  };
>>  EXPORT_SYMBOL_GPL(platform_bus_type);
>>  
>> +/** platform_bus_type_init - fill in a pseudo-platform-bus
>> +  * @bus: foriegn bus type
>> +  *
>> +  * This init is basically a selective memcpy that
>> +  * won't overwrite any user-defined attributes and
>> +  * only copies things that platform bus defines anyway
>> +  */
>> +void platform_bus_type_init(struct bus_type *bus)
>> +{
>> +	if (!bus->dev_attrs)
>> +		bus->dev_attrs = platform_bus_type.dev_attrs;
>> +	if (!bus->match)
>> +		bus->match = platform_bus_type.match;
>> +	if (!bus->uevent)
>> +		bus->uevent = platform_bus_type.uevent;
>> +	if (!bus->pm)
>> +		bus->pm = platform_bus_type.pm;
> 
> Watch out for things in "write only" memory here.  That could cause
> problems.

Pardon my ignorance (I'm quite new to kernel work), what do you mean
here? What memory could be "write only"?

> thanks,
> greg k-h

-- 
Employee of Qualcomm Innovation Center, Inc.
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