linux-kernel - Re: [PATCH] clk: imx: add CLK_GET_RATE_NOCACHE flag for i.MX8M composite clock

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Date:   Tue, 18 Dec 2018 13:35:41 +0000
From:   Anson Huang <anson.huang@....com>
To:     Lucas Stach <l.stach@...gutronix.de>
CC:     Fabio Estevam <festevam@...il.com>,
        "sboyd@...nel.org" <sboyd@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        Fabio Estevam <fabio.estevam@....com>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "mturquette@...libre.com" <mturquette@...libre.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] clk: imx: add CLK_GET_RATE_NOCACHE flag for i.MX8M
 composite clock

Hi, Lucas

From Anson's iPhone 6


> 在 2018年12月18日，18:40，Lucas Stach <l.stach@...gutronix.de> 写道：
> 
> Am Dienstag, den 18.12.2018, 08:24 -0200 schrieb Fabio Estevam:
>> Hi Anson,
>> 
>> On Tue, Dec 18, 2018 at 12:56 AM Anson Huang <anson.huang@....com>
>> wrote:
>>> 
>>> On i.MX8M, some of the bus clocks' rate could be changed in TF-A,
>> 
>> Do you mean ATF (ARM Trusted Firmware) instead?
> 
> TF-A is the name of the day for what was formerly known as ATF...
> 
> However I don't think that it's correct to just don't cache the clock
> settings. Normally the secure world firmware should not change any
> clock settings at runtime, or it would run into all kinds of conflicts
> with the clock driver. So there are probably some well known points in
> time like a suspend or resume event when the firmware might change
> clock settings, so we could instead use those to trigger an explicit
> invalidate of the clock caches with much lower overhead.
> 
> Regards,
> Lucas

There is bus-freq feature on imx8m which is to scale ddr clock, this is done in ARM Trusted Firmware, for some setpoints, the DDR PLL clock rate must be changed directly in TF-A, but its child clock like dram core is unaware in Linux kernel, so the clock rate will mismatch with hardware, since ddr related clocks will NOT used by any module in Linux kernel, so it will NOT introduce any conflict.

Regarding about the over head, yes, the change in common composite clock register has too many over head for other clocks, what if I ONLY have dram core clock to pass the CLK_GET_RATE_NOCACHE flag to register the composite clock?

Anson.