[PATCH 1/4] nvmem: rockchip-otp: Serialize reads

[Alexey Charkov]

The OTP controller is driven through a single set of registers holding a
state machine which has to be stepped through for every word read, yet
nothing keeps two readers out of each other's way. Concurrent reads
interleave, and the outcome is either a reader bailing out:

  rockchip-otp 2a580000.otp: timeout during read setup

or, worse, one of them silently taking delivery of the other's data.

Reading two cells in parallel from userspace on RK3576 reproduces both
within 150 iterations - 53 read errors and 9 corrupted results, the latter
either losing their first word or, in one case, ending in the two bytes
which belong to the other reader's cell - whereas the same reads issued
sequentially never fail. Concurrency is not hypothetical here, as six
thermal sensors source their trim values from the OTP and reach the driver
straight from asynchronous driver probing.

Guard the read path with a mutex. Reads are the only way into the hardware,
as the driver registers no write callback, and they always run in process
context, so a plain mutex spanning the whole clock-enable, read,
clock-disable sequence is enough.

Fixes: 755864feb729 ("nvmem: add Rockchip OTP driver")
Cc: stable at vger.kernel.org
Signed-off-by: Alexey Charkov <alchark at flipper.net>
---
 drivers/nvmem/rockchip-otp.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/nvmem/rockchip-otp.c b/drivers/nvmem/rockchip-otp.c
index 2c0feb036f3f..b034b7455016 100644
--- a/drivers/nvmem/rockchip-otp.c
+++ b/drivers/nvmem/rockchip-otp.c
@@ -12,6 +12,7 @@
 #include <linux/io.h>
 #include <linux/iopoll.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/nvmem-provider.h>
 #include <linux/reset.h>
 #include <linux/slab.h>
@@ -80,6 +81,8 @@ struct rockchip_otp {
 	void __iomem *base;
 	struct reset_control *rst;
 	const struct rockchip_data *data;
+	/* Serializes access to the OTP controller state machine */
+	struct mutex mutex;
 	struct clk_bulk_data clks[];
 };
 
@@ -272,6 +275,8 @@ static int rockchip_otp_read(void *context, unsigned int offset,
 	if (!otp->data || !otp->data->reg_read)
 		return -EINVAL;
 
+	guard(mutex)(&otp->mutex);
+
 	ret = clk_bulk_prepare_enable(otp->data->num_clks, otp->clks);
 	if (ret < 0) {
 		dev_err(otp->dev, "failed to prepare/enable clks\n");
@@ -431,6 +436,11 @@ static int rockchip_otp_probe(struct platform_device *pdev)
 
 	otp->data = data;
 	otp->dev = dev;
+
+	ret = devm_mutex_init(dev, &otp->mutex);
+	if (ret)
+		return ret;
+
 	otp->base = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(otp->base))
 		return dev_err_probe(dev, PTR_ERR(otp->base),

-- 
2.54.0