Re: [RFC PATCH 0/3] PM / Domains: Add support for devices that require multiple domains

[Marek Szyprowski]

Hi Stephen,

Thanks for pointing to my patches, but I would like to clarify a few things.

On 2016-11-24 03:30, Stephen Boyd wrote:
> On 11/22, Jon Hunter wrote:
> > On 22/11/16 18:26, Kevin Hilman wrote:
> > > Jon Hunter <jonathanh@xxxxxxxxxx> writes:
> > > > However, I would rather the client of
> > > > the power-domains specify which power-domains they require and
> > > > dynamically nested the power-domains at runtime. This is slightly
> > > > different to what I proposed in this RFC, but it is not really beyond
> > > > the bounds of what we support today IMO. What is missing is a means to
> > > > do this dynamically and not statically.
> > > >
> > > > By the way, I am not sure if you are suggesting that for devices that
> > > > may need multiple power-domains we should architect the driver
> > > > differently and split it up in some way such that we have a power-domain
> > > > per device. But for the case of the Tegra XHCI it is quite complex
> > > > because the driver loads firmware which runs on a micro-controller and
> > > > we need to manage the various power-domains that are used.
> > > IMO, constructing a network of new struct devices just to workaround
> > > limitations in the framework doesn't sound quite right either.
> > I agree.
> Marek is attempting to do this for the samsung clock
> controller[1] (patch #5 is informative).

You probably meant patch #3 / #4, which is a patch for Exynos 4412
( https://marc.info/?l=linux-arm-kernel&m=147731142926053&w=2 ).

Patch #5 is for Exynos 5433, which already has separate nodes for
each clock sub-controller, so there is no problem to add generic
power domains there (see multiple CMU nodes):

https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/tree/arch/arm64/boot/dts/exynos/exynos5433.dtsi#n261

>  From what I can tell
> they have one DT node for their clock controller because it's one
> register address space to control clocks. But, certain clocks
> exposed by that driver only work when certain power domains are
> enabled. For example, they have a clock controller that exposes
> clock signals for multimedia hardware blocks like video
> accelerators, gpus, and cameras. The clocks seem to have been
> placed inside different power domains for the multimedia hardware
> they're associated with, so there may be 10 or so power domains
> that need to be enabled at different times for different clocks
> to work. If the GPU power domain isn't enabled when the GPU
> clocks are touched by the driver, things break, etc.
>
> In the proposed patchset, we have the top-level clock controller
> node with subnodes for each power domain that needs to be
> associated with clocks inside these different multimedia blocks.

This is valid only for the Exynos4412 case (and not-yet-posted
Exynos5422), which has a single clock controller node and patch #4
added a sub-node for ISP clocks part (the only one which in fact
is in the other power domain).

> So we end up with one parent device and attached driver for the
> clock driver, and then that driver populates child nodes as
> devices and matches up clocks with child nodes while registering
> clks with clk_register(). Because we pass a dev pointer to
> clk_register, we associate different devices with different
> clocks all from the same top-level clock controller device
> driver. Then clk framework calls runtime_pm APIs with devices
> used during clk registration.

Right, this is how I did it for Exynos4412 case.

> Some of those devices don't have
> any driver bound to them, which feels odd.

Well, I don't get this. In the proposed patches each sub-node has
a separate driver, none is left without a driver.

> This seems like a case where we really want a better way to
> explicitly control power domains without making up subnodes and
> registering struct devices just to work around the one device to
> one genpd construct we have today. Maybe power domains just don't
> map to genpd though and that's the disconnect.

Having an API for full control over multiple power domain assigned to
a single device node might indeed solve somehow this problem, but as
long as runtime pm is tied to struct device, this will again end in
creating virtual sub-devices per each power domain to fit runtime pm
principles. However we might be able to avoid creating sub-nodes in
the device tree.

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland