linux-kernel - Re: [PATCH 1/5 v11] arm: omap: usb: ehci and ohci hwmod structures for omap4

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Date:	Wed, 28 Sep 2011 18:10:31 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Paul Walmsley <paul@...an.com>
Cc:	"Cousson, Benoit" <b-cousson@...com>,
	"Munegowda, Keshava" <keshava_mgowda@...com>,
	"parthab@...ia.ti.com" <parthab@...ia.ti.com>,
	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Balbi, Felipe" <balbi@...com>, "Gadiyar, Anand" <gadiyar@...com>,
	"sameo@...ux.intel.com" <sameo@...ux.intel.com>,
	"tony@...mide.com" <tony@...mide.com>,
	"Hilman, Kevin" <khilman@...com>,
	"johnstul@...ibm.com" <johnstul@...ibm.com>,
	"Sripathy, Vishwanath" <vishwanath.bs@...com>
Subject: Re: [PATCH 1/5 v11] arm: omap: usb: ehci and ohci hwmod structures
 for omap4

Hi Paul,

On Fri, Sep 23, 2011 at 7:31 AM, Paul Walmsley <paul@...an.com> wrote:

> The idea of the main_clk was not intended to be PRCM or OCP or even
> OMAP-specific.  It's just intended to represent a clock that is used to
> drive the register logic inside the IP block.  Therefore it must be
> enabled before any register access may occur.  Even if clock gating is
> handled by some higher-level interface (e.g., at the IP block level), the
> main_clk has a rate, so it also implies an upper limit on how quickly
> register operations can occur.  I suppose that all of the IP block's
> clocks could be "optional clocks," but we know that every IP block with
> registers requires at least one clock to work, and that should be the
> main_clk.

I am a bit confused about you comment on "main_clk".

>From hwmod related source code, main_clk is the function clock
of one module(hwmod), such as: on omap4, for uart3, main_clk is
uart3_fck.

But from[1] and [2] of omap4 PRM, we can find that interface clock
is required to provide register access instead of function clock.

This is a bit conflictive with what you description, so could you
give a further comments about main_clk, function clock and interface
clock?


[1], 23.3.4.2 Clock Configuration
	Each UART uses a 48-MHz functional clock for its logic
	and to generate external interface signals. Each UART
	uses an interface clock for register accesses.

[2], 3.1.1.1.1 Module Interface and Functional Clocks
	The interface clocks have the following characteristics:
	• They ensure proper communication between any module/subsystem
	and the interconnect.
	• In most cases, they supply the system interconnect interface
	and registers of the module.

thanks,
-- 
Ming Lei
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