[PATCH v2 2/2] firmware: exynos-acpm: allow use during system shutdown

["André Draszik" <andre.draszik@linaro.org>]

We need to access the PMIC during late system shutdown and at that time
we are not allowed to sleep anymore.

To make this case work, detect this condition and use busy waiting via
udelay() instead of usleep_range() in that situation.

The code isn't switched over to udelay() unconditionally so as to not
waste resources during normal operation. acpm_may_sleep() was heavily
inspired by the I2C subsystem's i2c_in_atomic_xfer_mode().

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
---
udelay(10) causes a checkpatch warning (it suggests to use
usleep_range() instead for usec >= 10), but that's exactly what we can
not do.
Reducing the udelay to be smaller will generally cause the loop to be
iterated more than once, which I wanted to avoid.
I could reflow the code to hide the actual value from checkpatch, e.g.
with the help of a local variable if that is preferred to ignoring the
checkpatch warning.
---
 drivers/firmware/samsung/exynos-acpm.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/samsung/exynos-acpm.c b/drivers/firmware/samsung/exynos-acpm.c
index 542eaff03f9e39422a8c5345ca75e05c1710a9ee..4f65f7ef39b5fdbf5bb10f6ee9ffb78c5e34d8b2 100644
--- a/drivers/firmware/samsung/exynos-acpm.c
+++ b/drivers/firmware/samsung/exynos-acpm.c
@@ -15,6 +15,8 @@
 #include <linux/firmware/samsung/exynos-acpm-protocol.h>
 #include <linux/io.h>
 #include <linux/iopoll.h>
+#include <linux/irqflags.h>
+#include <linux/kernel.h>
 #include <linux/ktime.h>
 #include <linux/mailbox/exynos-message.h>
 #include <linux/mailbox_client.h>
@@ -25,6 +27,7 @@
 #include <linux/of_address.h>
 #include <linux/of_platform.h>
 #include <linux/platform_device.h>
+#include <linux/preempt.h>
 #include <linux/slab.h>
 #include <linux/types.h>
 
@@ -273,6 +276,17 @@ static int acpm_get_rx(struct acpm_chan *achan, const struct acpm_xfer *xfer)
 	return 0;
 }
 
+/*
+ * When ACPM transfers happen very late, e.g. to access a PMIC when powering
+ * down, we can not sleep. We do want to sleep in the normal case, though, to
+ * avoid wasting CPU cycles!
+ */
+static bool acpm_may_sleep(void)
+{
+	return system_state <= SYSTEM_RUNNING ||
+		(IS_ENABLED(CONFIG_PREEMPT_COUNT) ? preemptible() : !irqs_disabled());
+}
+
 /**
  * acpm_dequeue_by_polling() - RX dequeue by polling.
  * @achan:	ACPM channel info.
@@ -300,7 +314,10 @@ static int acpm_dequeue_by_polling(struct acpm_chan *achan,
 			return 0;
 
 		/* Determined experimentally. */
-		usleep_range(20, 30);
+		if (!acpm_may_sleep())
+			udelay(10);
+		else
+			usleep_range(20, 30);
 	} while (ktime_before(ktime_get(), timeout));
 
 	dev_err(dev, "Timeout! ch:%u s:%u bitmap:%lx.\n",

-- 
2.49.0.395.g12beb8f557-goog