Message-ID: <1490963291.29083.69.camel@pengutronix.de>
Date:   Fri, 31 Mar 2017 14:28:11 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Dong Aisheng <dongas86@...il.com>
Cc:     Andrey Smirnov <andrew.smirnov@...il.com>,
        Shawn Guo <shawnguo@...nel.org>, yurovsky@...il.com,
        Fabio Estevam <fabio.estevam@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        rjw@...ysocki.net
Subject: Re: [PATCH v7 2/2] soc/imx: Add GPCv2 power gating driver

Hi Dong,

Am Samstag, den 01.04.2017, 12:10 +0800 schrieb Dong Aisheng:
[...]
> > If we need the domains to be up before the consumers, the only
> > way to deal with that is to select CONFIG_PM and
> > CONFIG_PM_GENERIC_DOMAINS from the platform, so we have proper probe
> > defer handling for consumer devices of the power domains.
> > 
> 
> A bit confuse about these words...

If those options are selected we get proper PROBE_DEFER handling for
consumers of the power domain, so probe order doesn't matter.

> > > Personally i'd be more like Rockchip's power domain implementation.
> > 
> > Why?
> > 
> > > See:
> > > arch/arm/boot/dts/rk3288.dtsi
> > > drivers/soc/rockchip/pm_domains.c
> > > Dcumentation/devicetree/bindings/soc/rockchip/power_domain.txt
> > > 
> > > How about refer to the Rockchip's way?
> > 
> > Why? We just changed the way how it's done for GPCv1, after more than 1
> > year of those patches being on the list. Why should we do it differently
> > for GPCv2?
> > 
> 
> Hmm?
> 
> I just thought your GPCv1 change was picked a few weeks ago (Feb 17 2017)
> and there's no current users in kernel of the new binding.
> That's why i come out of the idea if we could improve it before any users.
> 
> Maybe i made mistake?

The patches to change this have been out for over 1 year. I'm less than
motivated to change the binding again, after it has gone through the DT
review and has finally been picked up.

> See:
> commit 721cabf6c6600dbe689ee2782bc087270e97e652
> Author: Lucas Stach <l.stach@...gutronix.de>
> Date:   Fri Feb 17 20:02:44 2017 +0100
> 
>     soc: imx: move PGC handling to a new GPC driver
>     
>     This is an almost complete re-write of the previous GPC power gating control
>     code found in the IMX architecture code. It supports both the old and the new
>     DT binding, allowing more domains to be added later and generally makes the
>     driver easier to extend, while keeping compatibility with existing DTBs.
>     
>     As the result, all functionality regarding the power gating controller
>     gets removed from the IMX architecture GPC driver.  It keeps only the
>     IRQ controller code in the architecture, as this is closely coupled to
>     the CPU idle implementation.
>     
>     Signed-off-by: Lucas Stach <l.stach@...gutronix.de>
>     Signed-off-by: Shawn Guo <shawnguo@...nel.org>
> 
> > > 
> > > Then it could also address our issues and the binding would be
> > > still like:
> > > gpc: gpc@...a0000 {
> > >         compatible = "fsl,imx7d-gpc";
> > >         reg = <0x303a0000 0x1000>;
> > >         interrupt-controller;
> > >         interrupts = <GIC_SPI 87 IRQ_TYPE_LEVEL_HIGH>;
> > >         #interrupt-cells = <3>;
> > >         interrupt-parent = <&intc>;
> > > 
> > >         pgc {
> > >                 #address-cells = <1>;
> > >                 #size-cells = <0>;
> > > 
> > >                 pgc_pcie_phy: power-domain@...7_POWER_DOMAIN_PCIE_PHY {
> > >                         reg = <IMX7_POWER_DOMAIN_PCIE_PHY>;
> > >                         power-supply = <&reg_1p0d>;
> > > 			clocks = <xxx>;
> > >                 };
> > > 
> > > 		....
> > >         };
> > > };
> > > 
> > > It also drops #power-domain-cells and register domain by
> > > one provider with multi domains which is more align with HW.
> > > 
> > > How do you think of it?
> > 
> > How is this more aligned with the hardware? Both options are an
> > arbitrary abstraction chosen in the DT binding.
> > 
> 
> GPC is a Power Controller which controls multi power domains to subsystem
> by its different registers bits.
> e.g. PCIE/MIPI/USB HSIC PHY.
> • 0xC00 ~ 0xC3F: PGC for MIPI PHY
> • 0xC40 ~ 0xC7F: PGC for PCIE_PHY
> • 0xD00 ~ 0xD3F: PGC for USB HSIC PHY
> 
> So i thought it looks more like GPC is a power domain provider with multi
> domains support to different subsystems from HW point of view.
> 
> Isn't that true?

Linux and hardware devices are not required to match 1:1. There is
nothing that would make subdividing a single hardware device into
multiple ones bad style.

> 
> And there's also other two concerns:
> First, current genpd sysfs output still not include provider.
> e.g.
> root@...6qdlsolo:~# cat /sys/kernel/debug/pm_genpd/pm_genpd_summary 
> domain                          status          slaves
>     /device                                             runtime status
> ----------------------------------------------------------------------
> PU                              off-0           
>     /devices/soc0/soc/130000.gpu                        suspended
>     /devices/soc0/soc/134000.gpu                        suspended
>     /devices/soc0/soc/2204000.gpu                       suspended
>     /devices/soc0/soc/2000000.aips-bus/2040000.vpu      suspended
> ARM                             off-0           
> 
> I wonder it might be a bit mess once the provider is added while each
> domain is registered as a virtual provider.

The provider is a Linux device. Linux devices don't necessarily have to
correspond to hardware devices. There is no such rule.

> 
> Second, it sacrifices a bit performance when look-up PM domain in
> genpd_get_from_provider if every domain is a provider.
> 
The performance penalty of a list walk won't hurt us in the probe path,
where we are (re-)probing entire devices. There is a lot more going on
than a simple list walk.

> Though it is arguable that currently only 3 domains support on MX7,
> but who knows the future when it becomes much more.
> 
> However, i did see many exist users in kernel using one provider one
> domain way. Maybe i'm over worried and it's not big deal.

I see that one provider with multiple domains is used more often, that
doesn't means it's necessarily better. At least I haven't hear a
convincing argument on why the chosen implementation in the GPC driver
is worse than the alternative.

Regards,
Lucas

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