Re: [PATCH] slimbus: qcom-ngd-ctrl: drop the PM runtime reference after a sync transfer

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] slimbus: qcom-ngd-ctrl: runtime PM underflow in qcom_slim_ngd_xfer_msg_sync()
--

commit 95f275b622c76bd63160fe6491c0337ce83609c2
Author: Wentao Liang <vulab@iscas.ac.cn>

slimbus: qcom-ngd-ctrl: drop the PM runtime reference after a sync transfer

This commit adds a pm_runtime_put() call on the success path of
qcom_slim_ngd_xfer_msg_sync() to balance the pm_runtime_get_sync() call made
at the start of the transfer.

> diff --git a/drivers/slimbus/qcom-ngd-ctrl.c b/drivers/slimbus/qcom-ngd-ctrl.c
> index eb983b6d1815f..9bee4b376877e 100644
> --- a/drivers/slimbus/qcom-ngd-ctrl.c
> +++ b/drivers/slimbus/qcom-ngd-ctrl.c
> @@ -942,6 +942,9 @@ static int qcom_slim_ngd_xfer_msg_sync(struct slim_controller *ctrl,
>  		ret = -ETIMEDOUT;
>  		goto pm_put;
>  	}
> +
> +	pm_runtime_put(ctrl->dev);

[Severity: High]
Won't this underflow the runtime PM usage counter on ctrl->dev after every
successful synchronous transfer?

Every caller of qcom_slim_ngd_xfer_msg_sync() (qcom_slim_ngd_enable_stream(),
qcom_slim_ngd_disable_stream(), and qcom_slim_ngd_get_laddr()) allocates a
transaction ID via slim_alloc_txn_tid(ctrl, &txn).

When the reply or ACK (SLIM_USR_MC_ADDR_REPLY or SLIM_USR_MC_GENERIC_ACK)
arrives in qcom_slim_ngd_rx(), it invokes slim_msg_response(), which already
completes txn->comp (&done) and drops the runtime PM reference:

drivers/slimbus/qcom-ngd-ctrl.c:qcom_slim_ngd_rx() {
	...
	if (mc == SLIM_MSG_MC_REPLY_INFORMATION ||
	    mc == SLIM_MSG_MC_REPLY_VALUE || (mc == SLIM_USR_MC_ADDR_REPLY &&
	    mt == SLIM_MSG_MT_SRC_REFERRED_USER) ||
		(mc == SLIM_USR_MC_GENERIC_ACK &&
		 mt == SLIM_MSG_MT_SRC_REFERRED_USER)) {
		slim_msg_response(&ctrl->ctrl, &buf[4], buf[3], len - 4);
		pm_runtime_mark_last_busy(ctrl->ctrl.dev);
	}
}

drivers/slimbus/messaging.c:slim_msg_response() {
	...
	slim_free_txn_tid(ctrl, txn);
	memcpy(msg->rbuf, reply, len);
	if (txn->comp)
		complete(txn->comp);

	/* Remove runtime-pm vote now that response was received for TID txn */
	pm_runtime_mark_last_busy(ctrl->dev);
	pm_runtime_put_autosuspend(ctrl->dev);
}

Because slim_msg_response() drops the runtime PM reference when the response
is received, only the error and timeout paths in qcom_slim_ngd_xfer_msg_sync()
(where slim_msg_response() is not reached, as handled in commit 63c60a226c90)
should call pm_runtime_put(ctrl->dev). Calling pm_runtime_put(ctrl->dev) on
the success path releases the single pm_runtime_get_sync(ctrl->dev) reference
a second time, which can cause premature runtime suspension of the controller.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261104021424.1468172-1-vulab@iscas.ac.cn?part=1