Message-Id: <201108171600.37791.arnd@arndb.de>
Date:	Wed, 17 Aug 2011 16:00:37 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Kenneth Heitke <kheitke@...eaurora.org>, sdharia@...eaurora.org,
	David Brown <davidb@...eaurora.org>, bryanh@...eaurora.org,
	linux-arm-msm@...r.kernel.org, rdunlap@...otime.net,
	rmk+kernel@....linux.org.uk, john.stultz@...aro.org,
	akpm@...ux-foundation.org, ohad@...ery.com, gregkh@...e.de,
	stefanr@...6.in-berlin.de, lethal@...ux-sh.org,
	linville@...driver.com, zajec5@...il.com,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] slimbus: Linux driver framework for SLIMbus.

On Wednesday 17 August 2011, Mark Brown wrote:
> On Wed, 2011-08-17 at 12:42 +0200, Arnd Bergmann wrote:
> > > I'd expect that bringing the device out of reset is going to be largely
> > > unrelated to the host controller, it's going to be GPIOs, clocks and
> > > regulators.  The individual drivers are going to want to manage this
> > > stuff dynamically at runtime too.
> 
> > But it's even less related to the individual driver than to the host.
> 
> No, not at all - all the bus specifies is the two wire control
> interface, if a device on the bus requires power or anything else that's
> not something the CPU Slimbus (I keep wanting to typo that as
> Slumbus...) controller has any idea about. In this respect Slimbus is
> much more like I2C than USB where there's a standard provision for power
> even if embedded systems routinely ignore it.
>
> The device driver will know what power supplies and other signals the
> device has, and it will know how and when to use them. This can
> generally be done independently of the board with just some platform or
> device tree data to configure GPIOs.

Ok, I think you've managed to get through to me ;-)

> > The way I see this working is that something outside of the driver
> > should provide a way to enable each device in order for it to get
> > probed, and the driver's ->probe callback does a pm_runtime_get()
> > call when it wants to keep the device enabled.
> 
> Some pre-cooked off the shelf device wide power management is definitely
> useful for simple cases but I don't think that scales to high end
> devices - it's too binary. Like I said I really do want to have some
> transparent device model way of handling the simple cases but we need to
> leave room for devices which want to do more complicated things.
> 
> It also occurs to me that if we're supporting going down to cold with
> runtime PM anyway the kernel is going to have to be able to understand
> the idea that devices it already knows about are going to hotplug in and
> out while staying registered. If we're doing that then it seems like the
> bus is going to have pretty much all the concepts required for explicit
> registration anyway.

How about a mixed model then?

I can see three relevant cases to consider:

1. A simple potentially hotplugged device that registers itself to the bus
   can be automatically matched to the driver.
2. A device tree representation for hardwired devices that require
   something to happen in order to register to the bus (clock, regulator,
   ...).
3. A hardcoded list of devices on a slimbus host for stuff that is known
   to be there, e.g. on a PCI card that has its own driver and that
   also need some special setup as in case 2.

I think in all three cases, we should identify the device by its EA and
match that to the device driver. We create the slim_device and register
it to the bus as soon as one of the three above is found, but in case 2
and 3, the driver is responsible for the device to actually become active
on the bus before it's allowed to send any commands to it.

For the device tree binding, I would suggest defining a slimbus bus to
have #address-cells=1, #size-cells=0 and just put the EA into the reg
property. This is enough for the host driver to add create a
slim_device and match a driver to it. The driver can access all the
properties from the device_node (or platform_data in case of statically
defined devices). When the physical device shows up on the bus, it is
automatically associated with the existing slim_device.

	Arnd
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