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From:   Marek Szyprowski <m.szyprowski@samsung.com>
To:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        Mark Brown <broonie@kernel.org>,
        Dmitry Osipenko <digetx@gmail.com>
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Lucas Stach <l.stach@pengutronix.de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>, peron.clem@gmail.com,
        Nishanth Menon <nm@ti.com>, Stephen Boyd <sboyd@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Rafael Wysocki <rjw@rjwysocki.net>,
        linux-samsung-soc@vger.kernel.org,
        Chanwoo Choi <cw00.choi@samsung.com>
Subject: [PATCH 0/2] Fix regulators coupling for Exynos5800
Date:   Fri, 29 May 2020 14:49:38 +0200
Message-Id: <20200529124940.10675-1-m.szyprowski@samsung.com>
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CMS-TYPE: 201P
References: <CGME20200529124948eucas1p175379ead8afd1932f7b7ae61e35cf632@eucas1p1.samsung.com>
Sender: linux-kernel-owner@vger.kernel.org
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Hi!

This patchset is another attempt to fix the regulator coupling on
Exynos5800/5422 SoCs. Here are links to the previous attempts:

https://lore.kernel.org/linux-samsung-soc/20191008101709.qVNy8eijBi0LynOteWFMnTg4GUwKG599n6OyYoX1Abs@z/
https://lore.kernel.org/lkml/20191017102758.8104-1-m.szyprowski@samsung.com/
https://lore.kernel.org/linux-pm/cover.1589528491.git.viresh.kumar@linaro.org/
https://lore.kernel.org/linux-pm/20200528131130.17984-1-m.szyprowski@samsung.com/

The problem is with "vdd_int" regulator coupled with "vdd_arm" on Odroid
XU3/XU4 boards family. "vdd_arm" is handled by CPUfreq. "vdd_int" is
handled by devfreq. CPUfreq initialized quite early during boot and it
starts changing OPPs and "vdd_arm" value. Sometimes CPU activity during
boot goes down and some low-frequency OPPs are selected, what in turn
causes lowering "vdd_arm". This happens before devfreq applies its
requirements on "vdd_int". Regulator balancing code reduces "vdd_arm"
voltage value, what in turn causes lowering "vdd_int" value to the lowest
possible value. This is much below the operation point of the wcore bus,
which still runs at the highest frequency.

The issue was hard to notice because in the most cases the board managed
to boot properly, even when the regulator was set to lowest value allowed
by the regulator constraints. However, it caused some random issues,
which can be observed as "Unhandled prefetch abort" or low USB stability.

Adding more and more special cases to the generic code has been rejected,
so the only way to ensure the desired behavior on Exynos5800-based SoCs 
is to make a custom regulator coupler driver.

Best regards,
Marek Szyprowski


Patch summary:

Marek Szyprowski (2):
  regulator: extract voltage balancing code to separate function
  soc: samsung: Add simple voltage coupler for Exynos5800

 arch/arm/mach-exynos/Kconfig                  |  1 +
 drivers/regulator/core.c                      | 49 ++++++++-------
 drivers/soc/samsung/Kconfig                   |  3 +
 drivers/soc/samsung/Makefile                  |  1 +
 .../soc/samsung/exynos-regulator-coupler.c    | 59 +++++++++++++++++++
 include/linux/regulator/coupler.h             |  8 +++
 6 files changed, 101 insertions(+), 20 deletions(-)
 create mode 100644 drivers/soc/samsung/exynos-regulator-coupler.c

-- 
2.17.1