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Subject: [PATCH v2 5/6] nfit/ars: Allow root to busy-poll the ARS state
 machine
From:   Dan Williams <dan.j.williams@intel.com>
To:     linux-nvdimm@lists.01.org
Cc:     Erwin Tsaur <erwin.tsaur@oracle.com>, vishal.l.verma@intel.com,
        linux-kernel@vger.kernel.org
Date:   Fri, 15 Feb 2019 11:44:13 -0800
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The ARS implementation implements exponential back-off on the poll
interval to prevent high-frequency access to the DIMM / platform
interface. Depending on when the ARS completes the poll interval may
exceed the completion event by minutes. Allow root to reset the timeout
each time it probes the status. A one-second timeout is still enforced,
but root can otherwise can control the poll interval.

Reported-by: Erwin Tsaur <erwin.tsaur@oracle.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 drivers/acpi/nfit/core.c |    8 ++++++++
 drivers/acpi/nfit/nfit.h |    1 +
 2 files changed, 9 insertions(+)

diff --git a/drivers/acpi/nfit/core.c b/drivers/acpi/nfit/core.c
index 90312892093e..629cf91649d2 100644
--- a/drivers/acpi/nfit/core.c
+++ b/drivers/acpi/nfit/core.c
@@ -1333,6 +1333,13 @@ static ssize_t scrub_show(struct device *dev,
 	busy = test_bit(ARS_BUSY, &acpi_desc->scrub_flags)
 		&& !test_bit(ARS_CANCEL, &acpi_desc->scrub_flags);
 	rc = sprintf(buf, "%d%s", acpi_desc->scrub_count, busy ? "+\n" : "\n");
+	/* Allow an admin to poll the busy state at a higher rate */
+	if (busy && capable(CAP_SYS_RAWIO) && !test_and_set_bit(ARS_POLL,
+				&acpi_desc->scrub_flags)) {
+		acpi_desc->scrub_tmo = 1;
+		mod_delayed_work(nfit_wq, &acpi_desc->dwork, HZ);
+	}
+
 	mutex_unlock(&acpi_desc->init_mutex);
 	device_unlock(dev);
 	return rc;
@@ -3187,6 +3194,7 @@ static void acpi_nfit_scrub(struct work_struct *work)
 	else
 		notify_ars_done(acpi_desc);
 	memset(acpi_desc->ars_status, 0, acpi_desc->max_ars);
+	clear_bit(ARS_POLL, &acpi_desc->scrub_flags);
 	mutex_unlock(&acpi_desc->init_mutex);
 }
 
diff --git a/drivers/acpi/nfit/nfit.h b/drivers/acpi/nfit/nfit.h
index 897ce10192a0..d14bad687fb8 100644
--- a/drivers/acpi/nfit/nfit.h
+++ b/drivers/acpi/nfit/nfit.h
@@ -213,6 +213,7 @@ struct nfit_mem {
 enum scrub_flags {
 	ARS_BUSY,
 	ARS_CANCEL,
+	ARS_POLL,
 };
 
 struct acpi_nfit_desc {