linux-kernel - Re: [PATCH v2 4/4] clk: mediatek: Add USB clock support in MT8173 APMIXEDSYS

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Message-ID: <1435901927.3526.48.camel@mtksdaap41>
Date:	Fri, 3 Jul 2015 13:38:47 +0800
From:	James Liao <jamesjj.liao@...iatek.com>
To:	Daniel Kurtz <djkurtz@...omium.org>
CC:	Matthias Brugger <matthias.bgg@...il.com>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	srv_heupstream <srv_heupstream@...iatek.com>,
	Ricky Liang <jcliang@...omium.org>,
	Rob Herring <robh+dt@...nel.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	"open list:OPEN FIRMWARE AND..." <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<linux-mediatek@...ts.infradead.org>
Subject: Re: [PATCH v2 4/4] clk: mediatek: Add USB clock support in MT8173
 APMIXEDSYS

Hi Daniel,

On Wed, 2015-07-01 at 23:22 +0800, Daniel Kurtz wrote:
> This looks like 3 separate gate clocks in a chain, with a timing
> constraint: USB_LPF must be enabled 100 us after USB_TX.
> 
>   26MHz--> [GATE] --USB_TX--> [LPF] --USB_LPF--> [GATE] --USB_OUT-->
>             ^                  ^                  ^
>             +--------------+   |                  |
> AP_PLL_CON2.REF2USB_TX_EN -+   |                  |
> AP_PLL_CON2.REF2USB_TX_LPF_EN -+                  |
> AP_PLL_CON2.REF2USB_TX_OUT_EN --------------------+
> 
> 
> I think we can model the gate parts using a proper clock tree model
> and the existing clock gate semantics.
> I'm not sure the best way to model the delay; but in theory that could
> be handled by the clock user (USB driver).

Do you mean to create 3 hierarchical clocks (may be clock gates) to
model these clocks as the following ?

EN -- LPF -- OUT_EN

(EN is the parent of LPF, and LPF is the parent of EN)

If we model these 3 clocks like above, we can't prevent clock users to
enable OUT_EN directly, and there will be no delay between EN and LPF.

Or you have other suggestions to model these 3 clcoks?


Best regards,

James

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