linux-kernel - Re: [PATCH 0/5] Add Mediatek MT8173 subsystem clocks support

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Date:	Fri, 29 May 2015 08:23:45 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	James Liao <jamesjj.liao@...iatek.com>
Cc:	Matthias Brugger <matthias.bgg@...il.com>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	srv_heupstream@...iatek.com,
	Eddie Huang <eddie.huang@...iatek.com>,
	Henry Chen <henryc.chen@...iatek.com>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>,
	Daniel Kurtz <djkurtz@...omium.org>,
	Ricky Liang <jcliang@...omium.org>,
	Rob Herring <robh+dt@...nel.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH 0/5] Add Mediatek MT8173 subsystem clocks support

On Fri, May 29, 2015 at 10:47:29AM +0800, James Liao wrote:
> Hi Sascha,
> 
> > And really the driver matching "mediatek,mt8173-vencsys" should register
> > the necessary clocks and reset lines and call of_platform_populate on
> > the subnodes. The driver should also be a real driver, not something
> > matched by CLK_OF_DECLARE. The "mediatek,mt8173-vencsys" driver now has
> > the possibility to manage the toplevel vencsys unit, do runtime pm, turn
> > the whole thing off and on. Using CCF for abstracting these clocks may
> > be the right thing, but I believe that we should keep the code for the
> > toplevel vencsys register space together in a single file and not put
> > the clk bits in drivers/clk/mediatek/mt8173.c, the reset bits in
> > drivers/reset/ and the remaining misc stuff in drivers/soc/mediatek/.
> > 
> > So I think we should have a drivers/soc/mediatek/mtk-vencsys.c which
> > is a regular driver, calls clk_register() for its clocks, calls
> > reset_controller_register() for the reset bits, provides plain functions
> > for the remaining bits which are not handled by any Linux framework.
> > Finally of_platform_populate will register the child devices.
> > 
> > I showed this using the vencsys example, but it's the same for vdecsys,
> > vencltsys, imgsys and mmsys.
> 
> So you agree to manage these subsystem clocks in CCF, but they should be
> provided by their own (globalcon) drivers, right?

Yes. I previously got the impression that the subsystem clocks are not
directly associated to the larbs, but needed to be handled by the larb
code due to some side effect. Now that I saw that the larbs are directly
in the subsystem register space it all makes sense.

Note that the way Mediatek SoCs are designed around sub modules is bit
unusual and does not fit very well in the Linux directory structure.
Normally SoCs have a single clocks controller which controls all clocks
in the SoC. Then you often have a reset controller providing reset lines
in the SoC. In this case it's clear that the clk driver goes to
drivers/clk/, the reset controller driver to drivers/reset/. Mediatek
SoCs instead have several blocks, each with its own clock and reset
controller. Splitting each block up into parts in drivers/clk/ and
drivers/reset/ leads to quite a code fragmentation.
This is my opinion, it would be great to hear something from others.
Matthias? I'd like to avoid running into a direction that is not
acceptable in the end.

> 
> I have an implementation question. These subsystem clocks can't be
> implemented in CCF default clock-gate. So in our previous patches, we
> added a drivers/clk/mediate/clk-gate.c to implement new clock gate
> operations. Is it a good way to export mediatek/clk-gate.h (put it in
> include/linux/) for other drivers to implement their own clocks?

Give it a try. If that's not acceptable we can still shuffle this around
without much effort.

Sascha

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