Message-ID: <20110525023209.GA2535@b20223-02.ap.freescale.net>
Date:	Wed, 25 May 2011 10:32:09 +0800
From:	Richard Zhao <linuxzsc@...il.com>
To:	Colin Cross <ccross@...gle.com>
Cc:	Sascha Hauer <s.hauer@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-sh@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] Add a generic struct clk

On Tue, May 24, 2011 at 12:41:20PM -0700, Colin Cross wrote:
> On Tue, May 24, 2011 at 1:09 AM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> > On Tue, May 24, 2011 at 12:31:13AM -0700, Colin Cross wrote:
> >> On Mon, May 23, 2011 at 11:26 PM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> >> > On Mon, May 23, 2011 at 04:12:24PM -0700, Colin Cross wrote:
> >> >> >
> >> >> >   tglx's plan is to create a separate struct clk_hwdata, which contains a
> >> >> >   union of base data structures for common clocks: div, mux, gate, etc. The
> >> >> >   ops callbacks are passed a clk_hw, plus a clk_hwdata, and most of the base
> >> >> >   hwdata fields are handled within the core clock code. This means less
> >> >> >   encapsulation of clock implementation logic, but more coverage of
> >> >> >   clock basics through the core code.
> >> >>
> >> >> I don't think they should be a union, I think there should be 3
> >> >> separate private datas, and three sets of clock ops, for the three
> >> >> different types of clock blocks: rate changers (dividers and plls),
> >> >> muxes, and gates.  These blocks are very often combined - a device
> >> >> clock often has a mux and a divider, and clk_set_parent and
> >> >> clk_set_rate on the same struct clk both need to work.
> >> >
> >> > The idea is to being able to propagate functions to the parent. It's
> >> > very convenient for the implementation of clocks when they only
> >> > implement either a divider, a mux or a gate. Combining all of these
> >> > into a single clock leads to complicated clock trees and many different
> >> > clocks where you can't factor out the common stuff.
> >>
> >> That seems complicated.  You end up with lots of extra clocks (meaning
> >> more boilerplate in the platform files) that have no meaning in the
> >> system (what is the clock between the mux and the divider in Tegra's
> >> i2c1 clock, it can never feed any devices), and you have to figure out
> >> at the framework level when to propagate and when to error out.  I'm
> >> not even sure you can always find the right place to stop propagating
> >> - do you just keep going up until the set_parent callback succeeds, or
> >> exists, or what?
> >
> > For the set_parent there would be no propagating at all. For set_rate I
> > can imagine a flag in the generic clock which tells whether to propagate
> > set_rate or not.
> >
> >>
> >> I think you can still factor out all the common code if you treat each
> >> clock as a possible mux, divider, and gate combination.  Each part of
> >> the clock is still just as abstractable as before - you can set the
> >> rate_ops to be the generic single register divider implementation, and
> >> the gate ops to be the generic single bit enable implementation.  The
> >> idea of what a clock node is matches the HW design,
> >
> > The hardware design consists of only discrete rate changers (plls,
> > dividers), muxes and gates. These are the only building blocks
> > *every* hardware design has. I believe that many of the problems
> > the current implementations have are due to the multiple building
> > blocks stuffed into one clock. If you haven't already take a look
> > at my i.MX5 clock patches:
> >
> > http://thread.gmane.org/gmane.linux.ports.arm.kernel/113631
> >
> > They need changes to fit onto the current patches and the rate
> > propagation problem is not solved there, but the resulting clock
> > data files are really short and nice to read. Furthermore it's easy
> > to implement. Just look at the diagrams in the datasheet and go through
> > them.
> 
> Propagation is what I'm trying simplify, because it's impossible at
> the framework level to determine the right time to propagate, and the
> right time to return an error, and I don't like pushing it down to
> each clock implementation either, because then you need a "propagating
> clk gate" and a "non-propagating clk gate".
> 
> Your building blocks implement every op - clk_gate_ops implements
> set_rate as clk_parent_set_rate (won't that cause a deadlock on
> prepare_lock when it calls clk_set_rate?).  I'm suggesting breaking
> out the clock gate ops into a struct that only contains:
> enable
> disable
> prepare
> unprepare
> And a rate ops struct that contains:
> round_rate
> set_rate
> And a mux ops struct that contains
> set_parent
> 
> For a single HW clock that has a mux, a gate, and a divider, the
> existing implementation requires:
> INIT_CLK(..., clk_mux_ops)
> INIT_CLK(..., clk_div_ops)
> INIT_CLK(..., clk_gate_ops)
Not all clocks are like:
pll -> [mux] -> [gate] -> [divider] -> clk A
Some imx233 clocks are like:
pll -> [divider] -> -----------\
                              [mux] -> clk B
xtal -> [gate] -> [divider] -> /

Thanks
Richard
> which creates 3 clocks, and requires lots of propagation logic to
> figure out how to call clk_set_rate on the single clock that is
> exposed to the device, but not propagate it past the device clock if
> the device clock doesn't have a divider (propagating it up to an
> active pll feeding other devices results in disaster, at least on
> Tegra).  This combination doesn't seem to be common in your MX code,
> but _every_ Tegra device clock has these three parts.
> 
> With multiple smaller building blocks that can fit inside a clock, all
> you need is:
> INIT_CLK(..., clk_mux_ops, clk_div_ops, clk_gate_ops)
> You have one struct clk, which is exposed to the device and matches
> the datasheet.  If the clock has rate ops, clk_set_rate works with no
> propagation.  If it doesn't have a rate, clk_rate_ops is NULL, and the
> framework deal with propagation only in the case where it is really
> propagation - a child clock that requires changing a parent clock.
> The block abstraction is still in place, there are just 3 slots for
> blocks within each clock.
> 
> Using a flag to mark clocks as "non-propagatable" is also not correct
> - propagatability is a feature of the parent, not the child.
> 
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> 
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