linux-kernel - Re: [v3] x86/tsc: Unset TSC_KNOWN_FREQ and TSC

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Message-ID: <20200129155910.GJ32742@smile.fi.intel.com>
Date:   Wed, 29 Jan 2020 17:59:10 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Hans de Goede <hdegoede@...hat.com>,
        vipul kumar <vipulk0511@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        linux-kernel@...r.kernel.org, Stable <stable@...r.kernel.org>,
        Srikanth Krishnakar <Srikanth_Krishnakar@...tor.com>,
        Cedric Hombourger <Cedric_Hombourger@...tor.com>,
        x86@...nel.org, Len Brown <len.brown@...el.com>,
        Vipul Kumar <vipul_kumar@...tor.com>
Subject: Re: [v3] x86/tsc: Unset TSC_KNOWN_FREQ and TSC_RELIABLE flags on
 Intel Bay Trail SoC

On Wed, Jan 29, 2020 at 05:53:53PM +0200, Andy Shevchenko wrote:
> On Wed, Jan 29, 2020 at 04:13:39PM +0100, Thomas Gleixner wrote:
> > Hans de Goede <hdegoede@...hat.com> writes:
> > > On 29-01-2020 15:14, Andy Shevchenko wrote:
> > >>> The only one which is possibly suspicious here is this line:
> > >>>
> > >>>   * 0111:   25 * 32 /  9  =  88.8889 MHz
> > >>>
> > >>> The SDM says 88.9 MHz for this one.
> > 
> > I trust math more than the SDM :)
> > 
> > >> Anyway it seems need to be fixed as well.
> > >> 
> > >> Btw, why we are mentioning 20 / 6 and 28 / 6 when arithmetically
> > >> it's the same as 10 / 3 and 14 / 3?
> > >
> > > I copied the BYT values from Thomas' email and I guess he did not
> > > get around to simplifying them, I'll use the simplified versions
> > > for my patch.
> > 
> > Too tired, too lazy :)
> > 
> > Andy, can you please make sure that people inside Intel who can look
> > into the secrit documentation confirm what we are aiming for?
> > 
> > Ideally they should provide the X-tal frequency and the mult/div pair
> > themself :)
> 
> So, I don't have access to the CPU core documentation (and may be will not be
> given), nevertheless I dug a bit to what I have for Cherrytrail. So, the XTAL
> is 19.2MHz, which becomes 100MHz and 1600MHz by some root PLL, then, the latter
> two frequencies are being used by another PLL to provide a reference clock (*)
> to PLL which derives CPU clock.

> *) According to colleagues of mine it's a fixed rate source.

One more thing.

Depends on SKU it may be 400MHz, 320MHz, 200MHz or 333MHz.
(I guess these values should be kinda references in the table)

> That's all what I have.

-- 
With Best Regards,
Andy Shevchenko