linux-kernel - Re: [PATCH 3.18 052/144] ARM: dra7xx: Fix counter frequency drift for AM572x errata i856

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Date:   Fri, 9 Nov 2018 10:59:34 -0500
From:   lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Lokesh Vutla <lokeshvutla@...com>,
        Tony Lindgren <tony@...mide.com>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 3.18 052/144] ARM: dra7xx: Fix counter frequency drift
 for AM572x errata i856

On Thu, Nov 08, 2018 at 01:50:23PM -0800, Greg Kroah-Hartman wrote:
> 3.18-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> [ Upstream commit afc9d590b8a150cfeaac0078ef5de6fb21a5ea6a ]
> 
> Errata i856 for the AM572x (DRA7xx) points out that the 32.768KHz external
> crystal is not enabled at power up.  Instead the CPU falls back to using
> an emulation for the 32KHz clock which is SYSCLK1/610.  SYSCLK1 is usually
> 20MHz on boards so far (which gives an emulated frequency of 32.786KHz),
> but can also be 19.2 or 27MHz which result in much larger drift.
> 
> Since this is used to drive the master counter at 32.768KHz * 375 /
> 2 = 6.144MHz, the emulated speed for 20MHz is of by 570ppm, or about 43
> seconds per day, and more than the 500ppm NTP is able to tolerate.
> 
> Checking the CTRL_CORE_BOOTSTRAP register can determine if the CPU
> is using the real 32.768KHz crystal or the emulated SYSCLK1/610, and
> by known that the real counter frequency can be determined and used.
> The real speed is then SYSCLK1 / 610 * 375 / 2 or SYSCLK1 * 75 / 244.

I certainly can't see a problem including it.  Of course I doubt there
really are any DRA7 systems running that old a kernel (certainly the
beaglebone X15 seems to require a newer kernel to even have a DTB
included).  The RuggedCom RX1400 might still be running 3.14 kernel,
but already has the patch applied (because I did it).

So certainly nothing wrong with adding the patch, but I doubt anyone
will notice.

-- 
Len Sorensen