linux-kernel - Re: [RFC PATCH 0/2] clk: rockchip: leave npll for DCLK

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Date:	Wed, 14 Jan 2015 14:16:15 -0800
From:	Mike Turquette <mturquette@...aro.org>
To:	Heiko Stübner <heiko@...ech.de>,
	"Kever Yang" <kever.yang@...k-chips.com>, dianders@...omium.org
Cc:	sonnyrao@...omium.org, addy.ke@...k-chips.com, cf@...k-chips.com,
	dkl@...k-chips.com, huangtao@...k-chips.com,
	linux-rockchip@...ts.infradead.org, tomeu.vizoso@...labora.com,
	sboyd@...eaurora.org,
	"Ian Campbell" <ijc+devicetree@...lion.org.uk>,
	devicetree@...r.kernel.org, "Dmitry Torokhov" <dtor@...omium.org>,
	linux-kernel@...r.kernel.org, "Kumar Gala" <galak@...eaurora.org>,
	"Jianqun" <jay.xu@...k-chips.com>,
	"Rob Herring" <robh+dt@...nel.org>,
	"Pawel Moll" <pawel.moll@....com>,
	"Chris Zhong" <zyw@...k-chips.com>,
	"Mark Rutland" <mark.rutland@....com>,
	"Russell King" <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 0/2] clk: rockchip: leave npll for DCLK_VOP0(HDMI) only

Quoting Heiko Stübner (2015-01-08 14:30:01)
> Hi Kever,
> 
> Am Montag, 17. November 2014, 22:55:36 schrieb Kever Yang:
> > To support all kinds of frequency requirement for HDMI on rk3288,
> > we need a PLL that can change rate at run time.
> > 
> > There are some discussion before at [0], I think we can just leave
> > the npll for HDMI(DCLK_VOP0) used to make it simple.
> > 
> > Comments are welcome.
> 
> I think I said it in private somewhere already, but just so it's also 
> available publically:
> 
> I don't think customizing/limiting the clock usage like this will fly, 
> especially as this would require each and every rk3288 board to use vop0 for 
> hdmi and vop1 for other stuff.
> 
> With the new rk3288 Firefly devboard this concern already becomes reality. 
> There a vga converter is connected to VOP0, which leaves only vop1 for hdmi if 
> one wants to support the vga connection.
> 
> 
> From our discussion about this problem I remember that the missing clock 
> frequencies only affected more esotheric screen resolutions, so personally I'm 
> not this much concerned an would like to wait till we find a better solution to 
> the problem.

Ack. We shouldn't have to limit the possible hardware configurations in
software just to keep things simple. This points to a deficiency in the
clock framework. This is a common concern: how to change a clock
frequency for one user without exploding all of the other users.

Do you think Tomeu's constraints API[0] might be a step in the right
direction for you?

[0] http://lkml.kernel.org/r/<1421071809-17402-3-git-send-email-tomeu.vizoso@...labora.com>

Regards,
Mike

> 
> 
> Heiko
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