linux-kernel - Re: [PATCH v5 1/2] dt-bindings: brcm: clocks: add binding for brcmstb-cpu-clk-div

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Message-ID: <20170206225914.GJ25384@codeaurora.org>
Date:   Mon, 6 Feb 2017 14:59:14 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Markus Mayer <markus.mayer@...adcom.com>,
        Markus Mayer <code@...yer.net>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>,
        Linux Clock List <linux-clk@...r.kernel.org>,
        Power Management List <linux-pm@...r.kernel.org>,
        Device Tree List <devicetree@...r.kernel.org>,
        ARM Kernel List <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 1/2] dt-bindings: brcm: clocks: add binding for
 brcmstb-cpu-clk-div

On 02/03, Florian Fainelli wrote:
> On 02/03/2017 12:06 PM, Stephen Boyd wrote:
> > On 02/01, Markus Mayer wrote:
> >> On 20 January 2017 at 16:52, Stephen Boyd <sboyd@...eaurora.org> wrote:
> >>>
> >>> Are these properties used? Please don't put these types of
> >>> details in DT.
> >>
> >> Yeah, unfortunately they are. Luckily, I think the issue can be
> >> resolved quite easily, because the user of those properties isn't
> >> involved in this series.
> >>
> >> They are currently being used by a clock driver
> >> ("drivers/clk/clk-brcmstb.c") that hasn't been upstreamed yet. I
> >> performed some code archeology. While I wasn't 100% successful in
> >> tracking down the origins of this interface, it looks like it was
> >> designed this way a while back (4+ years or so), probably before
> >> device tree best practices were developed or, at least, before they
> >> were widely known.
> >>
> >> So, what I can do is to remove the properties from the official
> >> binding. (I'll send an update to that effect shortly.) Once the
> >> binding is accepted upstream, we can work on fixing up the design of
> >> clk-brcmstb.c, so it doesn't rely on these properties anymore (and
> >> derives them from the compatible string instead), and then proceed to
> >> upstream that, as well.
> > 
> > Ok. Sounds like some cleanup needs to be done on the way
> > upstream.
> > 
> >>> This register really looks like some offset in something larger.
> >>> Is there some clock controller? What's the hw block at
> >>> 0xf03e2000? Maybe I already asked this.
> >>
> >> It looks this way, but in this case, looks are deceiving. The address
> >> and the length are really correct the way they are.
> >>
> >> This memory area holds a range of only loosely related configuration
> >> registers. It's called the Bus Interface Unit Register Set and deals
> >> with configuring the CPU in general. At address 0xf03e257c, there
> >> happens to be the clock divider register we need, and it's really just
> >> one register, i.e. 4 bytes.
> > 
> > We've seen this style of hardware design before. I'd prefer we
> > make the "Bus Interface Unit Register Set" into one device node
> > and have a driver probe for it that registers this clock. If
> > other things need to be controlled in there then the driver will
> > do more than just register one clock, possibly hooking into
> > multiple frameworks. The compatible string can indicate which SoC
> > it is if the divider register offset changes or if the register
> > layout is a total free for all.
> 
> We already have another piece of drive code that manipulates registers
> in the Bus Interface Unit located in drivers/soc/bcm/brcmstb/biuctrl.c
> and which has little to nothing to do with the CPU's clock ratio. And
> actually another one being submitted that deals with the CPU's
> read-ahead cache. I would very much prefer we keep all of them separate
> and dealing with just the register offset they need to do, but that does
> not mean the Device Tree binding has to look that way though.
> 
> The binding for the BIUCTRL register made it here:
> 
> Documentation/devicetree/bindings/arm/bcm/brcm,brcmstb.txt
> 
> so we should re-use that, and have a small piece of clock provided that
> just uses the relevant register range within that larger register space
> and provide the CLOCK_RATIO. Does that work?
> 

Ok. That's fine. The existing binding will be updated to include
this new subnode then for the clock component?

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