linux-kernel - RE: [PATCH] clk: imx: lpcg: write twice when writing lpcg regs

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Date:   Tue, 10 Sep 2019 02:47:59 +0000
From:   Anson Huang <anson.huang@....com>
To:     Dong Aisheng <dongas86@...il.com>, Stephen Boyd <sboyd@...nel.org>,
        Peng Fan <peng.fan@....com>
CC:     "festevam@...il.com" <festevam@...il.com>,
        "mturquette@...libre.com" <mturquette@...libre.com>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        Aisheng Dong <aisheng.dong@....com>,
        Peng Fan <peng.fan@....com>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>, Jacky Bai <ping.bai@....com>,
        Abel Vesa <abel.vesa@....com>,
        "linux-clk@...r.kernel.org" <linux-clk@...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>
Subject: RE: [PATCH] clk: imx: lpcg: write twice when writing lpcg regs



> On Sat, Sep 7, 2019 at 9:47 PM Stephen Boyd <sboyd@...nel.org> wrote:
> >
> > Quoting Peng Fan (2019-08-27 01:17:50)
> > > From: Peng Fan <peng.fan@....com>
> > >
> > > There is hardware issue that:
> > > The output clock the LPCG cell will not turn back on as expected,
> > > even though a read of the IPG registers in the LPCG indicates that
> > > the clock should be enabled.
> > >
> > > The software workaround is to write twice to enable the LPCG clock
> > > output.
> > >
> > > Signed-off-by: Peng Fan <peng.fan@....com>
> >
> > Does this need a Fixes tag?
> 
> Not sure as it's not code logic issue but a hardware bug.
> And 4.19 LTS still have not this driver support.

Looks like there is an errata for this issue, and Ranjani just sent a patch for review internally,

Back-to-back LPCG writes can be ignored by the LPCG register due to a 
HW bug. The writes need to be separated by atleast 4 cycles of the gated clock.
The workaround is implemented as follows:
1. For clocks running greater than 50MHz no delay is required as the 
delay in accessing the LPCG register is sufficient.
2. For clocks running greater than 23MHz, a read followed by the write 
will provide the sufficient delay.
3. For clocks running below 23MHz, LPCG is not used.

Need double check?

Anson.