Re: [PATCH v2 09/11] accel/rocket: add devfreq support

Thank you for your contribution! Sashiko AI review found 5 potential issue(s) to consider:
- [High] pm: power leak due to async pm_runtime_put_autosuspend() cancelled by pm_runtime_disable() in teardown
- [High] pm: SError crash on resume due to ignored return value of dev_pm_opp_set_rate()
- [High] pm: SError on power-on due to unsafe rounding up of the boot rate
- [High] race: UAF / NULL pointer dereference in rocket_npu_restore_boot_rate() racing with shutdown
- [High] race: double-free of devfreq and OPP table due to concurrent sysfs unbinds
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commit e0300730a57da6df01c259dea64c48000cbfedae
Author: Igor Paunovic [off-list ref]

accel/rocket: add devfreq support

This commit introduces devfreq support to scale the NPU clock rate based on
core utilization. It uses a single devfreq device for all cores and runtime
resumes all cores when raising the clock rate to prevent hardware SError.

[ ... ]

+void rocket_devfreq_suspend(struct rocket_device *rdev)
+{
+	struct rocket_devfreq *rdevfreq = &rdev->devfreq;
+
+	if (!rdevfreq->devfreq)
+		return;
+
+	guard(mutex)(&rdevfreq->lock);
+
+	if (!rdevfreq->cores_held)
+		return;
+
+	rocket_devfreq_set_rate(rdev, rdevfreq->boot_freq);
+	rdevfreq->cores_held = false;
+	rocket_devfreq_release_all(rdev);
+}

[Severity: High]
What happens if rocket_devfreq_set_rate() fails here? The return value is
ignored, and cores_held is cleared unconditionally. If the rate wasn't
lowered successfully but we drop references and allow the core to suspend,
will this cause an asynchronous SError when the core later powers on with a
raised clock?

[ ... ]

+	freq = rdev->npu_boot_rate;
+	opp = devfreq_recommended_opp(dev, &freq, 0);
+	if (IS_ERR(opp)) {
+		ret = dev_err_probe(dev, PTR_ERR(opp),
+				    "no OPP covers the %lu Hz boot rate\n",
+				    rdev->npu_boot_rate);
+		goto err_remove_table;
+	}
+
+	/*
+	 * From here on the boot rate is the OPP it maps to. The raw rate is
+	 * the firmware's number and need not be in the table at all.
+	 */
+	rdevfreq->boot_freq = freq;
+
+	/*
+	 * Program the supply for the rate the NPU is already running, so that
+	 * the regulator is not switched off underneath it by
+	 * regulator_late_cleanup(). That OPP is the boot rate rounded up to
+	 * the table, so this may raise the clock, and a raised clock is only
+	 * ever programmed with every core held: the same rule as ->target().
+	 */
+	ret = rocket_devfreq_hold_all(rdev);
+	if (ret) {
+		dev_pm_opp_put(opp);
+		dev_err_probe(dev, ret,
+			      "cannot resume the NPU cores to set the initial OPP\n");
+		goto err_remove_table;
+	}
+	ret = dev_pm_opp_set_opp(dev, opp);
+	rocket_devfreq_release_all(rdev);

[Severity: High]
Is it safe to round up npu_boot_rate to the nearest OPP and then allow the
device to suspend? The commit message mentions that an island powered up
while the clock is above the rate the bootloader left will take an SError.
If devfreq_recommended_opp() rounds the frequency up, and we set it and
drop references via rocket_devfreq_release_all(), won't the next power-on
trigger the SError because the clock is now higher than the safe
bootloader rate?

[ ... ]

+void rocket_devfreq_fini(struct rocket_device *rdev)
+{
+	struct rocket_devfreq *rdevfreq = &rdev->devfreq;
+	struct device *dev;
+
+	if (!rdevfreq->devfreq)
+		return;
+
+	dev = rdevfreq->owner->dev;
+
+	devfreq_remove_device(rdevfreq->devfreq);
+	rdevfreq->devfreq = NULL;
+
+	/*
+	 * Lower the clock before letting go of the cores, not after: a core
+	 * that suspends while the clock is still raised would be unable to
+	 * come back.
+	 */
+	scoped_guard(mutex, &rdevfreq->lock) {
+		if (rdevfreq->cores_held) {
+			rocket_devfreq_set_rate(rdev, rdevfreq->boot_freq);
+			rdevfreq->cores_held = false;
+			rocket_devfreq_release_all(rdev);
+		}
+	}

[Severity: High]
Could concurrent sysfs unbinds cause a double-free here? Since
rocket_remove() in rocket_drv.c calls rocket_devfreq_fini() without a
driver-wide lock, multiple unbinds could pass the if (!rdevfreq->devfreq)
check simultaneously and try to free rdevfreq->devfreq and destroy the
mutex twice.

Also, similar to rocket_devfreq_suspend(), if rocket_devfreq_set_rate()
fails, won't unconditionally dropping references here lead to an SError on
resume?

[ ... ]

[Severity: High]
If rocket_devfreq_fini() uses pm_runtime_put_autosuspend() under the hood
to release PM references via rocket_devfreq_release_all(), does
rocket_core_fini() immediately disable runtime PM? As per the PM subsystem
guide, pm_runtime_disable() cancels any pending asynchronous suspend. Will
this leave the device permanently powered on and cause a power leak?

[ ... ]

[Severity: High]
Is there a race condition between rocket_npu_restore_boot_rate() and
rocket_shutdown()? If a job finishes and the PM workqueue calls
rocket_device_runtime_suspend() concurrently with system shutdown:

CPU1 in rocket_shutdown():
rocket_devfreq_fini(rdev); (sets rdevfreq->owner = NULL and frees OPP table)

CPU2 in rocket_npu_restore_boot_rate() sees rdev->devfreq.owner as valid
just before it is set to NULL, and calls rocket_devfreq_set_boot_rate(rdev)
which does:

dev_pm_opp_set_rate(rdevfreq->owner->dev, rdevfreq->boot_freq);

Could this result in a NULL pointer dereference or use-after-free of the
OPP table since the devfreq owner has been destroyed?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260922080114.44662-1-royalnet026@gmail.com?part=9