Message-Id: <20251013-automatic-clocks-v1-0-72851ee00300@linaro.org>
Date: Mon, 13 Oct 2025 21:51:29 +0100
From: Peter Griffin <peter.griffin@...aro.org>
To: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, 
 Conor Dooley <conor+dt@...nel.org>, Alim Akhtar <alim.akhtar@...sung.com>, 
 André Draszik <andre.draszik@...aro.org>, 
 Tudor Ambarus <tudor.ambarus@...aro.org>, 
 Michael Turquette <mturquette@...libre.com>, 
 Stephen Boyd <sboyd@...nel.org>, Sam Protsenko <semen.protsenko@...aro.org>, 
 Sylwester Nawrocki <s.nawrocki@...sung.com>, 
 Chanwoo Choi <cw00.choi@...sung.com>
Cc: Will McVicker <willmcvicker@...gle.com>, 
 Krzysztof Kozlowski <krzk@...nel.org>, devicetree@...r.kernel.org, 
 linux-arm-kernel@...ts.infradead.org, linux-samsung-soc@...r.kernel.org, 
 linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org, 
 Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>, 
 kernel-team@...roid.com, Peter Griffin <peter.griffin@...aro.org>
Subject: [PATCH 0/9] Implement hardware automatic clock gating (HWACG) for
 gs101

Hi folks,

This series addresses an issue with Samsung Exynos based upstream clock driver
whereby the upstream clock driver sets all the clock gates into "manual mode"
(which uses a bit that is documented as reserved in the gate registers).

Another issue with the current "manual clock gating" approach upstream is
there are many bus/interconnect clocks whose relationships to the IPs
are not well documented or defined in the specs. When adding a new CMU until
now we have tried to label these clocks appropriately with CLK_IS_CRITICAL and
CLK_IGNORE_UNUSED but doing so is both error prone and time consuming. If
your lucky disabling a critical bus clock causes an immediate hang. Other
clocks however aren't so obvious and show up through random instability
some period of time later.

Fortunately each CMU (at least on newer Exynos) provides a "hardware
automatic clock gating" HWACG feature that is used by the downstream
Samsung clock drivers. Hardware automatic clock gating uses a hardware
interface between the CMU and IP to control all clocks required by the
IP. This interface is called Q-channel, and is part of the Arm AMBA low
power interface specification [1].

The advantage of using this Qchannel hardware interface for
enabling/disabling the clocks is that it takes care of all clocks
(including bus/interconnect) ones for the IP automatically thereby reducing
the dynamic power.

Whilst each clock component (GATE, MUX, DIV, QCH etc) has a HWACG enable
bit there are also some "global enable override" bits for the entire CMU in
the CMU_CONTROLLER_OPTION register.

This series makes use of those "global enable" override bits to enable auto
clock mode for the entire CMU and every component within it. Through
experimentation we can see that setting the "manual mode" reserved gate bit
on a particular gate register overides the global enable bits. So the code
is updated accordingly not to do that.

Auto clock mode has been implemented as a "opt in" by setting a new
auto_clock_gate flag in the CMU static data. The intention is existing
platforms in manual mode should not be effected by any of these changes.

If auto_clock_mode flag is set and the option_offset field is specified
then the global enable override bits will be written for the
CMU (to avoid relying on any prior bootstage configuration). Again if auto
mode is enabled the code no longer sets MANUAL and clears HWACG bits on
each gate register.

To have dynamic root clock gating (drcg) of bus components and memclk
enabled, it is required to set the bus_component_drcg and memclk registers
in the the correspondingly named sysreg controller. If auto clock mode is
enabled the clock driver will now attempt to get the sysreg syscon via the
samsung,sysreg property (as used by other Exynos drivers upstream) and set
the registers accordingly. The suspend/resume code paths are also updated
to handle saving/restoring registers using a regmap. Note cmu_top is an
exception and does not have a corresondingly named sysreg_top.

As all clock gates are currently exposed in the gs101 drivers and DT, we
continue to register all of these gates in auto clock mode, but with some new
samsung_auto_clk_gate_ops. As clk enable and clk disable are now handled by
Q-channel interface the .enable and .disable implementations are
no-ops. However by using some CMU qchannel debug registers we can report
the current clock status (enabled or disabled) of every clock gate in the
system. This has the nice effect of still being able to dump the entire
clock tree from /sys/kernel/debug/clk/clk_summary and see a live view of
every auto clock in the system.

With the infrastructure in place, all the CMUs registerred in clk-gs101 are
now updated to enable auto clock mode. From dumping
/sys/kernel/debug/clk/clk_summary it is possible to see that after enabling
auto clock mode approximately 305 clocks are enabled, and 299 are now
disabled. This number goes up and down a bit by 3-5 clocks just on a idle
system sat at a console. As the CLK_IGNORE_UNUSED and CLK_IS_CRITICAL flags
lose any meaning in auto clock mode they are removed. It is now also
possible to boot without the clk_ignore_unused kernel command line property
for the first time! Prior to enabling auto clock mode 586 clocks were
enabled, and 17 disabled (in part due to having to boot with
clk_ignore_unused flag). To ensure compatability with older DTs the
resource size is now checked and an error issued if the DT needs updating to be
compatible with auto clock mode.

For future CMUs in gs101 I propose we continue to expose all gates, but
register the CMU in "auto mode". For new device drivers or updates to
existing dt bindings related to clocks to support gs101 I suggest we only
use the "obviously correct" clock(s). By "obviously correct" I mean a clock
has the IP name in the clock register name, but not try to deduce other
obsucurely named bus/interconnect clocks which will now all be handled
automatically. Note it is still possible to test whether the "obviously
correct" clock is indeed correct by putting the individual gate in manual
mode and disabling the clock (e.g. by using devmem). 

Note: As everything here will go via one of Krzysztof's trees I've sent it
as one series.

regards,

Peter

[1] https://documentation-service.arm.com/static/5f915e69f86e16515cdc3b3e?token=

Signed-off-by: Peter Griffin <peter.griffin@...aro.org>
---
Peter Griffin (9):
      dt-bindings: soc: samsung: exynos-sysreg: add gs101 hsi0 and misc compatibles
      dt-bindings: clock: google,gs101-clock: add samsung,sysreg property as required
      arm64: dts: exynos: gs101: add sysreg_misc and sysreg_hsi0 nodes
      arm64: dts: exynos: gs101: fix clock module unit reg sizes
      arm64: dts: exynos: gs101: fix sysreg_apm reg property
      arm64: dts: exynos: gs101: add samsung,sysreg property to CMU nodes
      clk: samsung: Implement automatic clock gating mode for CMUs
      clk: samsung: gs101: Enable auto_clock_gate mode for each gs101 CMU
      clk: samsung: gs101: remove CLK_IGNORE_UNUSED and CLK_IS_CRITICAL flags

 .../bindings/clock/google,gs101-clock.yaml         |  23 ++-
 .../soc/samsung/samsung,exynos-sysreg.yaml         |   4 +
 arch/arm64/boot/dts/exynos/google/gs101.dtsi       |  36 +++--
 drivers/clk/samsung/clk-exynos-arm64.c             |  47 +++++-
 drivers/clk/samsung/clk-exynos4.c                  |   6 +-
 drivers/clk/samsung/clk-exynos4412-isp.c           |   4 +-
 drivers/clk/samsung/clk-exynos5250.c               |   2 +-
 drivers/clk/samsung/clk-exynos5420.c               |   4 +-
 drivers/clk/samsung/clk-gs101.c                    | 167 ++++++++++++++-------
 drivers/clk/samsung/clk.c                          | 161 ++++++++++++++++++--
 drivers/clk/samsung/clk.h                          |  49 +++++-
 11 files changed, 412 insertions(+), 91 deletions(-)
---
base-commit: 4a71531471926e3c391665ee9c42f4e0295a4585
change-id: 20251008-automatic-clocks-249ab60f62ce

Best regards,
-- 
Peter Griffin <peter.griffin@...aro.org>

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