Re: [RFC PATCH 00/16] Centralize the ownership of power rail votes for Qualcomm clocks

[Taniya Das]

On 10/14/2026 5:49 PM, Konrad Dybcio wrote:
> On Qualcomm SoCs, both the PLLs and other types of clocks alike require
> specific voltage rail levels to operate in-spec, but through the
> separation and decentralization of clock and voltage votes, these
> requirements are often not met and are never explicitly stated.
> This results in clocks being undervolted, or at the very least a state
> we're unable to reason about.
> 
> The devices consuming these clocks very rarely have their own
> requirements in that regard. (1)
> 
> This series moves the frequency-power_domain-perf_state mappings inside
> the clock drivers, ensuring a single, trusted source of truth is
> established and that the silicon is no longer driven at out-of-spec
> conditions.
> 
> To illustrate the nuances involved, here's a couple examples of hidden
> dependencies, as found on SM8650:
> * video_cc_mvs0_clk requires LOW_SVS to run at 900 MHz, but the
>   PLL it sources the signal from requires SVS (higher corner) to
>   produce that frequency. Similarly, to produce 1140 MHz, the clock
>   requires SVS, but the PLL then requires SVS_L1.
>   Analogous story for mvs1_clk.
> * gpu_cc_gmu_clk requires MX to be at SVS to output 625 MHz, the PLL
>   requires SVS_L1 to produce that frequency.
> * A number of camera clocks follow the same pattern.
> * Additionally, some clocks like cam_cc_cci_{0,1,2}_clk require
>   MMCX to run at LOW_SVS, but the PLL has the additional requirement
>   of keeping MXC (a wholly different rail) at SVS_L1.
> 
> There's a broad agreement that a consumer should only have to care
> about the clock it consumes, and that configuring its parents is the
> responsibility of the CCF. Requiring the consumers to care about the
> above cases (as is the expectation today) is a clear violation of
> that contract.
> 
> A clock controller may have on the order of hundreds of clocks, each
> one with its own frequency-power map, and potentially all of them
> could be similarly affected.
> 
> Recently, some drivers (Iris, CAMSS) have attempted to circumvent these
> problems by holding a direct reference to the power rails and managing
> the votes manually, through placing direct votes on each rail via an
> OPP table. That however has a number of drawbacks:
> * Because more than just one clock needs to be scaled at runtime, the
>   OPP table either has to contain NUM_CLOCKS * NUM_RATES to handle
>   that (which blows up pretty quickly), assume the worst case (and
>   waste power), or ignore that problem (and undervolt the clock
>   controller), instead of just being able to call set_rate() on each
>   clock as needed
> * The data ends up being dispersed across various consumers, even
>   though it logically belongs to the clock controller, making it
>   absolutely unauditable
> * It's horrendously error-prone because of the number of clocks
>   involved, the depth of the clock tree and the fact that at the end
>   of the day, one still has to consult the clock controller
>   documentation
> 
> The approach proposed here also happens to more naturally fit into the
> process of how an SoC is brought up - clock controllers are always
> the very first pieces that need to be complete before other IP blocks
> can be brought up. And because of (1), enclosing the frequency-corner
> data within the clock drivers takes a weight off of everyone else's
> shoulders.
> 
> On a related topic, the GDSCs are modeled as children of *all* power
> domains consumed by a clock controller at once. GDSCs are on/off switches
> with some fancy logic attached to them and may need some (or none) rails
> enabled to power on or off while meeting the hardware timing
> requirements. Voting on them was never quite right in the first place
> and is once again only a side-effect of the clocks' power requirements
> not being handled centrally.
> 
> This series removes the vote propagation and the overly broad parent-
> child links, resulting in dropping unnecessary/incorrect votes that are
> now encapsulated by the clk-dvfs infrastructure. Big emphasis on (1)
> once again.
> 
> The implementation is as follows:
> 
> Each clock describes the rail levels it needs at each frequency. The
> new clk-dvfs helper combines these requirements and submits the
> resulting votes through GenPD and RPMHPD. Votes remain held while a
> clock is prepared, and rate or parent changes reserve the new
> requirements before any clocks are reprogrammed. This also requires CCF
> to report programming failures and OPP to retry failed transitions
> instead of relying on stale cached state.
> 
>                 Consumer: select rate, prepare clock
>                                    |
>                                    v
>         +------------------------ CCF ----------------------+
>         | Walk the clock tree and reserve resources          |
>         | before programming rates or parents                |
>         +--------------------------------------------------+
>                                    |
>                                    v
>  +------------------------------------------------------------------+
>  |                                                                    |
>  |    GPLL9 ------> SDCC2 RCG -------> SDCC2 apps clock               |
>  |      |               |                                             |
>  |      |               |                         Other GCC clocks    |
>  |      v               v                              |              |
>  |  +------------+  +------------+                     v              |
>  |  | PLL's own  |  | RCG's own  |          Per-clock requirements    |
>  |  | rate table |  | rate table |                         |          |
>  |  +-----+------+  +-----+------+                         |          |
>  |        |               |                                |          |
>  |        | CX            | CX + MX                        |          |
>  |        v               v                                v          |
>  |    +--------------------------------------------------------+      |
>  |    |                  Clock-owned DVFS                        |    |
>  |    |                                                          |    |
>  |    | Prepared votes + temporary transition votes              |    |
>  |    |                          |                               |    |
>  |    |               Maximum requirement per rail               |    |
>  |    +-----------+--------------+--------------+--------------+      |
>  |                |              |              |                     |
>  |            CX voter      CX_AO voter     MX voter                  |
>  +----------------+-------------+-------------+---------------------+
>                    |              |              |
>                    v              v              v
>        +----------------------- GenPD ----------------------+
>        |      Aggregate GCC votes with other consumers       |
>        +---------------------------------------------------+
>                                   |
>                                   v
>                             +-----------+
>                             |   RPMHPD  |
>                             +-----------+
> 
> Straightening out all of the above lets us tackle another problem
> with the upstream description of the power grid - the dependency
> between various rails. Most notably:
> * CX does NOT need to follow the performance state of MMCX
> * MX/MXA/MXC and CX are co-dependent only on some platforms (in coarse
>   terms, SM8350 and before)
> 
> However, the main part of series needs to land first to ensure that
> dropping those relationships doesn't remove some wrongful votes the
> system relied on to stay functional. Resolving that will save power and
> finally ensure total stability as far as the providers are concerned.
> 
> SM8650 is used as an example in this series, but the conversion will
> have to be done for all Qualcomm platforms. That will follow soon
> after the patches are merged. DFS clocks are left out of this RFC to
> reduce noise (the remaining diff is not particularly large but also
> requires boilerplate changes to bus drivers).
> 
> dt-bindings changes are not included since it's an RFC.
> 
> Signed-off-by: Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx>
> ---
> Konrad Dybcio (16):
>       clk: Propagate rate programming failures
>       clk: Acquire resources for clock transitions
>       OPP: Retry failed default single-clock transitions
>       pmdomain: core: Support named indexed virtual attachments
>       pmdomain: core: Support subdomains without pstate propagation
>       pmdomain: core: Support indexed power-domain attachment lists
>       clk: qcom: Add clock-owned GenPD DVFS aggregation
>       clk: qcom: dvfs: Add debugfs
>       clk: qcom: clk-alpha-pll: Add DVFS (un)prepare
>       clk: qcom: clk-alpha-pll: Vote for the PLL calibration frequency
>       clk: qcom: gdsc: Make GDSC parenting optional
>       clk: qcom: gdsc: Link required power domains
>       [DNM] arm64: dts: qcom: sm8650: Describe clock controller power rails
>       [DNM] clk: qcom: Enable clk-dvfs on SM8650
>       [DNM] arm64: dts: qcom: sm8650: Drop required-opps
>       [DNM] pmdomain: qcom: rpmhpd: Stop forwarding MMCX pstate to CX
> 
>  arch/arm64/boot/dts/qcom/sm8650.dtsi        |  62 +--
>  drivers/base/power/common.c                 |  24 +-
>  drivers/clk/Makefile                        |   1 +
>  drivers/clk/clk.c                           | 793 +++++++++++++++++++++++-----
>  drivers/clk/clk_resource_test.c             | 376 +++++++++++++
>  drivers/clk/clk_test.c                      | 194 ++++++-
>  drivers/clk/qcom/Makefile                   |   1 +
>  drivers/clk/qcom/camcc-sm8650.c             | 282 ++++++++++
>  drivers/clk/qcom/clk-alpha-pll.c            | 317 ++++++++++-
>  drivers/clk/qcom/clk-dvfs.c                 | 635 ++++++++++++++++++++++
>  drivers/clk/qcom/clk-dvfs.h                 |  41 ++
>  drivers/clk/qcom/clk-rcg2.c                 |  93 +++-
>  drivers/clk/qcom/clk-regmap.h               |   6 +
>  drivers/clk/qcom/common.c                   |  28 +-
>  drivers/clk/qcom/common.h                   |   3 +
>  drivers/clk/qcom/dispcc-sm8550.c            | 328 ++++++++++--
>  drivers/clk/qcom/gcc-sm8650.c               | 261 +++++++++
>  drivers/clk/qcom/gdsc.c                     | 154 +++++-
>  drivers/clk/qcom/gdsc.h                     |   7 +
>  drivers/clk/qcom/gpucc-sm8650.c             | 101 +++-
>  drivers/clk/qcom/videocc-sm8550.c           | 135 ++++-
>  drivers/opp/core.c                          |  72 ++-
>  drivers/opp/opp.h                           |   2 +
>  drivers/pmdomain/core.c                     |  60 ++-
>  drivers/pmdomain/qcom/rpmhpd.c              |  14 +-
>  include/dt-bindings/clock/qcom,sm8650-gcc.h |   2 +
>  include/linux/clk-provider.h                |  39 ++
>  include/linux/pm_domain.h                   |  33 +-
>  28 files changed, 3734 insertions(+), 330 deletions(-)
> ---
> base-commit: 4eb2586bd40db27d77cc5b724ca9386ed0c71d53
> change-id: 20261001-topic-qc_clk_dvfs-ef907f059611
> 
> Best regards,
> --  
> Konrad Dybcio <konrad.dybcio@xxxxxxxxxxxxxxxx>
> 


This seems to be similar proposal which was there earlier

https://lore.kernel.org/all/1532196902-28570-5-git-send-email-tdas@xxxxxxxxxxxxxx/


-- 
Thanks,
Taniya Das