Re: [PATCH] remoteproc: mediatek: Break lock dependency to `prepare_lock`

[Tzung-Bi Shih <tzungbi@kernel.org>]

On Mon, Jan 05, 2026 at 03:16:33PM -0700, Mathieu Poirier wrote:
> On Mon, Dec 29, 2025 at 04:31:46AM +0000, Tzung-Bi Shih wrote:
> > `scp_ipi_send` acquires `prepare_lock` via `clk_prepare_enable` while
> > the caller often holds `ec_dev->lock` (e.g., `cros_ec_cmd_xfer`).  The
> > reverse dependency exists where `clk_prepare` can trigger operations
> > that eventually take `ec_dev->lock` (e.g., via sysfs/regulator/genpd).
> 
> What operation would that be?  Please be specific so that I can trace the code.

The chain is discovered by lockdep: &ec_dev->lock -> prepare_lock ->
&genpd->mlock -> ... -> kn->active#2 -> &ec_dev->lock.

-> #6 (&ec_dev->lock){+.+.}-{3:3}:
       __mutex_lock_common
       mutex_lock_nested
       cros_ec_cmd_xfer
       cros_ec_cmd_xfer_status
       cros_usbpd_charger_get_port_status
       cros_usbpd_charger_get_prop
       power_supply_get_property
       power_supply_show_property
       power_supply_uevent
       dev_uevent
       uevent_show
       dev_attr_show
       sysfs_kf_seq_show
       kernfs_seq_show
       seq_read_iter
       kernfs_fop_read_iter
       vfs_read
-> #5 (kn->active#2){++++}-{0:0}:
       kernfs_drain
       __kernfs_remove
       kernfs_remove_by_name_ns
       sysfs_remove_file_ns
       device_del
       __device_link_del
       device_links_driver_bound
       driver_bound
-> #4 (device_links_lock){+.+.}-{3:3}:
       __mutex_lock_common
       mutex_lock_nested
       device_link_remove
       _regulator_put
       regulator_put
       devm_regulator_release
...
-> #1 (&genpd->mlock){+.+.}-{3:3}:
       __mutex_lock_common
       mutex_lock_nested
       genpd_lock_mtx
       genpd_runtime_resume
       __rpm_callback
       rpm_callback
       rpm_resume
       __pm_runtime_resume
       clk_core_prepare
       clk_prepare
-> #0 (prepare_lock){+.+.}-{3:3}:
       __lock_acquire
       lock_acquire
       __mutex_lock_common
       mutex_lock_nested
       clk_prepare
       scp_ipi_send
       scp_send_ipi
       mtk_rpmsg_send
       rpmsg_send
       cros_ec_pkt_xfer_rpmsg

> > Move clock prepare / unprepare operations to remoteproc prepare() /
> > unprepare() callbacks to break the lock dependency from `ec_dev->lock`
> > to `prepare_lock`.
> 
> With the information presented to me, I don't see how doing that changes
> anything.  @prepare_lock is simply held for a longer period of time.  

In prepare() callback, the clock becomes prepared and prepare_lock won't be
held after that.