ps2-gpio: use ktime for IRQ timekeeping

ps2-gpio: use ktime for IRQ timekeeping

[Danilo Krummrich]

Changes since v1
================
  - add patch to refactor struct ps2_gpio_data for clear separation between
    RX and TX
  - make all variables for IRQ timekeeping per-port and initialize them in
    ps2_gpio_open()

This patch series implements the usage of ktime for IRQ timekeeping to
overcome:

(1) The resolution limitations of jiffies.
(2) Potential spurious IRQs generated by gpio controllers.

Besides that, based on the newly implemented timekeeping, it fixes a wrongly
suspected extra clock cycle for TX transfers and a race condition when
starting an immediate TX transfer based on data received from an RX transfer.

Danilo Krummrich (4):
      input: ps2-gpio: refactor struct ps2_gpio_data
      input: ps2-gpio: use ktime for IRQ timekeeping
      input: ps2-gpio: remove tx timeout from ps2_gpio_irq_tx()
      input: ps2-gpio: don't send rx data before the stop bit

 drivers/input/serio/ps2-gpio.c | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------------
 1 file changed, 116 insertions(+), 64 deletions(-)

[PATCH 1/4] input: ps2-gpio: refactor struct ps2_gpio_data

[Danilo Krummrich]

Refactor struct ps2_gpio_data in order to clearly separate RX and TX
state data.

This change intends to increase code readability and does not bring any
functional change.

Signed-off-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
---
 drivers/input/serio/ps2-gpio.c | 70 ++++++++++++++++++++--------------
 1 file changed, 41 insertions(+), 29 deletions(-)

diff --git a/drivers/input/serio/ps2-gpio.c b/drivers/input/serio/ps2-gpio.c
index 8970b49ea09a..5f68505dd473 100644
--- a/drivers/input/serio/ps2-gpio.c
+++ b/drivers/input/serio/ps2-gpio.c
@@ -52,13 +52,17 @@ struct ps2_gpio_data {
 	struct gpio_desc *gpio_data;
 	bool write_enable;
 	int irq;
-	unsigned char rx_cnt;
-	unsigned char rx_byte;
-	unsigned char tx_cnt;
-	unsigned char tx_byte;
-	struct completion tx_done;
-	struct mutex tx_mutex;
-	struct delayed_work tx_work;
+	struct ps2_gpio_data_rx {
+		unsigned char cnt;
+		unsigned char byte;
+	} rx;
+	struct ps2_gpio_data_tx {
+		unsigned char cnt;
+		unsigned char byte;
+		struct completion complete;
+		struct mutex mutex;
+		struct delayed_work work;
+	} tx;
 };
 
 static int ps2_gpio_open(struct serio *serio)
@@ -73,7 +77,7 @@ static void ps2_gpio_close(struct serio *serio)
 {
 	struct ps2_gpio_data *drvdata = serio->port_data;
 
-	flush_delayed_work(&drvdata->tx_work);
+	flush_delayed_work(&drvdata->tx.work);
 	disable_irq(drvdata->irq);
 }
 
@@ -85,9 +89,9 @@ static int __ps2_gpio_write(struct serio *serio, unsigned char val)
 	gpiod_direction_output(drvdata->gpio_clk, 0);
 
 	drvdata->mode = PS2_MODE_TX;
-	drvdata->tx_byte = val;
+	drvdata->tx.byte = val;
 
-	schedule_delayed_work(&drvdata->tx_work, usecs_to_jiffies(200));
+	schedule_delayed_work(&drvdata->tx.work, usecs_to_jiffies(200));
 
 	return 0;
 }
@@ -98,12 +102,12 @@ static int ps2_gpio_write(struct serio *serio, unsigned char val)
 	int ret = 0;
 
 	if (in_task()) {
-		mutex_lock(&drvdata->tx_mutex);
+		mutex_lock(&drvdata->tx.mutex);
 		__ps2_gpio_write(serio, val);
-		if (!wait_for_completion_timeout(&drvdata->tx_done,
+		if (!wait_for_completion_timeout(&drvdata->tx.complete,
 						 msecs_to_jiffies(10000)))
 			ret = SERIO_TIMEOUT;
-		mutex_unlock(&drvdata->tx_mutex);
+		mutex_unlock(&drvdata->tx.mutex);
 	} else {
 		__ps2_gpio_write(serio, val);
 	}
@@ -111,12 +115,20 @@ static int ps2_gpio_write(struct serio *serio, unsigned char val)
 	return ret;
 }
 
+static inline struct ps2_gpio_data *
+to_ps2_gpio_data(struct delayed_work *dwork)
+{
+	struct ps2_gpio_data_tx *txd = container_of(dwork,
+						    struct ps2_gpio_data_tx,
+						    work);
+
+	return container_of(txd, struct ps2_gpio_data, tx);
+}
+
 static void ps2_gpio_tx_work_fn(struct work_struct *work)
 {
 	struct delayed_work *dwork = to_delayed_work(work);
-	struct ps2_gpio_data *drvdata = container_of(dwork,
-						    struct ps2_gpio_data,
-						    tx_work);
+	struct ps2_gpio_data *drvdata = to_ps2_gpio_data(dwork);
 
 	enable_irq(drvdata->irq);
 	gpiod_direction_output(drvdata->gpio_data, 0);
@@ -130,8 +142,8 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 	int rxflags = 0;
 	static unsigned long old_jiffies;
 
-	byte = drvdata->rx_byte;
-	cnt = drvdata->rx_cnt;
+	byte = drvdata->rx.byte;
+	cnt = drvdata->rx.cnt;
 
 	if (old_jiffies == 0)
 		old_jiffies = jiffies;
@@ -220,8 +232,8 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 	old_jiffies = 0;
 	__ps2_gpio_write(drvdata->serio, PS2_CMD_RESEND);
 end:
-	drvdata->rx_cnt = cnt;
-	drvdata->rx_byte = byte;
+	drvdata->rx.cnt = cnt;
+	drvdata->rx.byte = byte;
 	return IRQ_HANDLED;
 }
 
@@ -231,8 +243,8 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 	int data;
 	static unsigned long old_jiffies;
 
-	cnt = drvdata->tx_cnt;
-	byte = drvdata->tx_byte;
+	cnt = drvdata->tx.cnt;
+	byte = drvdata->tx.byte;
 
 	if (old_jiffies == 0)
 		old_jiffies = jiffies;
@@ -284,7 +296,7 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 		}
 
 		drvdata->mode = PS2_MODE_RX;
-		complete(&drvdata->tx_done);
+		complete(&drvdata->tx.complete);
 
 		cnt = 1;
 		old_jiffies = 0;
@@ -305,9 +317,9 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 	cnt = 1;
 	old_jiffies = 0;
 	gpiod_direction_input(drvdata->gpio_data);
-	__ps2_gpio_write(drvdata->serio, drvdata->tx_byte);
+	__ps2_gpio_write(drvdata->serio, drvdata->tx.byte);
 end:
-	drvdata->tx_cnt = cnt;
+	drvdata->tx.cnt = cnt;
 	return IRQ_HANDLED;
 }
 
@@ -403,11 +415,11 @@ static int ps2_gpio_probe(struct platform_device *pdev)
 	/* Tx count always starts at 1, as the start bit is sent implicitly by
 	 * host-to-device communication initialization.
 	 */
-	drvdata->tx_cnt = 1;
+	drvdata->tx.cnt = 1;
 
-	INIT_DELAYED_WORK(&drvdata->tx_work, ps2_gpio_tx_work_fn);
-	init_completion(&drvdata->tx_done);
-	mutex_init(&drvdata->tx_mutex);
+	INIT_DELAYED_WORK(&drvdata->tx.work, ps2_gpio_tx_work_fn);
+	init_completion(&drvdata->tx.complete);
+	mutex_init(&drvdata->tx.mutex);
 
 	serio_register_port(serio);
 	platform_set_drvdata(pdev, drvdata);
-- 
2.35.1

Re: [PATCH 1/4] input: ps2-gpio: refactor struct ps2_gpio_data

[Dmitry Torokhov]

Hi Danilo,

On Tue, Feb 15, 2022 at 05:02:05PM +0100, Danilo Krummrich wrote:

> +static inline struct ps2_gpio_data *
> +to_ps2_gpio_data(struct delayed_work *dwork)
> +{
> +	struct ps2_gpio_data_tx *txd = container_of(dwork,
> +						    struct ps2_gpio_data_tx,
> +						    work);
> +
> +	return container_of(txd, struct ps2_gpio_data, tx);
> +}
> +
>  static void ps2_gpio_tx_work_fn(struct work_struct *work)
>  {
>  	struct delayed_work *dwork = to_delayed_work(work);
> -	struct ps2_gpio_data *drvdata = container_of(dwork,
> -						    struct ps2_gpio_data,
> -						    tx_work);

This can simply be written as:

	struct ps2_gpio_data *drvdata = container_of(dwork,
						     struct ps2_gpio_data,
						     tx.work);

No need to resubmit unless you disagree - I can change it on my side.

Thanks.

-- 
Dmitry

Re: [PATCH 1/4] input: ps2-gpio: refactor struct ps2_gpio_data

[Danilo Krummrich]

Hi Dmitry,

On Tue, Feb 15, 2022 at 01:54:46PM -0800, Dmitry Torokhov wrote:
> Hi Danilo,
> 
> On Tue, Feb 15, 2022 at 05:02:05PM +0100, Danilo Krummrich wrote:
> 
> > +static inline struct ps2_gpio_data *
> > +to_ps2_gpio_data(struct delayed_work *dwork)
> > +{
> > +	struct ps2_gpio_data_tx *txd = container_of(dwork,
> > +						    struct ps2_gpio_data_tx,
> > +						    work);
> > +
> > +	return container_of(txd, struct ps2_gpio_data, tx);
> > +}
> > +
> >  static void ps2_gpio_tx_work_fn(struct work_struct *work)
> >  {
> >  	struct delayed_work *dwork = to_delayed_work(work);
> > -	struct ps2_gpio_data *drvdata = container_of(dwork,
> > -						    struct ps2_gpio_data,
> > -						    tx_work);
> 
> This can simply be written as:
> 
> 	struct ps2_gpio_data *drvdata = container_of(dwork,
> 						     struct ps2_gpio_data,
> 						     tx.work);
> 
> No need to resubmit unless you disagree - I can change it on my side.
Thanks, please do so.

The tx and rx members of struct ps2_gpio_data can then be anonymous structs.
Do you mind changing that too? Or should I resubmit?
> 
> Thanks.
> 
> -- 
> Dmitry

- Danilo

Re: [PATCH 1/4] input: ps2-gpio: refactor struct ps2_gpio_data

[Dmitry Torokhov]

On Tue, Feb 15, 2022 at 11:23:17PM +0100, Danilo Krummrich wrote:
> Hi Dmitry,
> 
> On Tue, Feb 15, 2022 at 01:54:46PM -0800, Dmitry Torokhov wrote:
> > Hi Danilo,
> > 
> > On Tue, Feb 15, 2022 at 05:02:05PM +0100, Danilo Krummrich wrote:
> > 
> > > +static inline struct ps2_gpio_data *
> > > +to_ps2_gpio_data(struct delayed_work *dwork)
> > > +{
> > > +	struct ps2_gpio_data_tx *txd = container_of(dwork,
> > > +						    struct ps2_gpio_data_tx,
> > > +						    work);
> > > +
> > > +	return container_of(txd, struct ps2_gpio_data, tx);
> > > +}
> > > +
> > >  static void ps2_gpio_tx_work_fn(struct work_struct *work)
> > >  {
> > >  	struct delayed_work *dwork = to_delayed_work(work);
> > > -	struct ps2_gpio_data *drvdata = container_of(dwork,
> > > -						    struct ps2_gpio_data,
> > > -						    tx_work);
> > 
> > This can simply be written as:
> > 
> > 	struct ps2_gpio_data *drvdata = container_of(dwork,
> > 						     struct ps2_gpio_data,
> > 						     tx.work);
> > 
> > No need to resubmit unless you disagree - I can change it on my side.
> Thanks, please do so.
> 
> The tx and rx members of struct ps2_gpio_data can then be anonymous structs.
> Do you mind changing that too? Or should I resubmit?

I will.

Thanks.

-- 
Dmitry

[PATCH 2/4] input: ps2-gpio: use ktime for IRQ timekeeping

[Danilo Krummrich]

Using jiffies for the IRQ timekeeping is not sufficient for two reasons:

(1) Usually jiffies have a resolution of 1ms to 10ms. The IRQ intervals
    based on the clock frequency of PS2 protocol specification (10kHz -
    16.7kHz) are between ~60us and 100us only. Therefore only those IRQ
    intervals can be detected which are either at the end of a transfer
    or are overly delayed. While this is sufficient in most cases, since
    we have quite a lot of ways to detect faulty transfers, it can
    produce false positives in rare cases: When the jiffies value
    changes right between two interrupt that are in time, we wrongly
    assume that we missed one or more clock cycles.

(2) Some gpio controllers (e.g. the one in the bcm283x chips) may generate
    spurious IRQs when processing interrupts in the frequency given by PS2
    devices.

Both issues can be fixed by using ktime resolution for IRQ timekeeping.

However, it is still possible to miss clock cycles without detecting
them. When the PS2 device generates the falling edge of the clock signal
we have between ~30us and 50us to sample the data line, because after
this time we reach the next rising edge at which the device changes the
data signal already. But, the only thing we can detect is whether the
IRQ interval is within the given period. Therefore it is possible to
have an IRQ latency greater than ~30us to 50us, sample the wrong bit on
the data line and still be on time with the next IRQ. However, this can
only happen when within a given transfer the IRQ latency increases
slowly.

___            ______            ______            ______            ___
   \          /      \          /      \          /      \          /
    \        /        \        /        \        /        \        /
     \______/          \______/          \______/          \______/

    |-----------------|                 |--------|
         60us/100us                      30us/50us

Signed-off-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
---
 drivers/input/serio/ps2-gpio.c | 80 ++++++++++++++++++++++++++++------
 1 file changed, 66 insertions(+), 14 deletions(-)

diff --git a/drivers/input/serio/ps2-gpio.c b/drivers/input/serio/ps2-gpio.c
index 5f68505dd473..246a583986e9 100644
--- a/drivers/input/serio/ps2-gpio.c
+++ b/drivers/input/serio/ps2-gpio.c
@@ -19,6 +19,7 @@
 #include <linux/of.h>
 #include <linux/jiffies.h>
 #include <linux/delay.h>
+#include <linux/timekeeping.h>
 
 #define DRIVER_NAME		"ps2-gpio"
 
@@ -44,6 +45,29 @@
 
 #define PS2_CMD_RESEND		0xfe
 
+/* The PS2 protocol specifies a clock frequency between 10kHz and 16.7kHz,
+ * therefore the maximal interrupt interval should be 100us and the minimum
+ * interrupt interval should be ~60us. Let's allow +/- 20us for frequency
+ * deviations and interrupt latency.
+ *
+ * The data line must be samples after ~30us to 50us after the falling edge,
+ * since the device updates the data line at the rising edge.
+ *
+ * ___            ______            ______            ______            ___
+ *    \          /      \          /      \          /      \          /
+ *     \        /        \        /        \        /        \        /
+ *      \______/          \______/          \______/          \______/
+ *
+ *     |-----------------|                 |--------|
+ *          60us/100us                      30us/50us
+ */
+#define PS2_CLK_FREQ_MIN_HZ		10000
+#define PS2_CLK_FREQ_MAX_HZ		16700
+#define PS2_CLK_MIN_INTERVAL_US		((1000 * 1000) / PS2_CLK_FREQ_MAX_HZ)
+#define PS2_CLK_MAX_INTERVAL_US		((1000 * 1000) / PS2_CLK_FREQ_MIN_HZ)
+#define PS2_IRQ_MIN_INTERVAL_US		(PS2_CLK_MIN_INTERVAL_US - 20)
+#define PS2_IRQ_MAX_INTERVAL_US		(PS2_CLK_MAX_INTERVAL_US + 20)
+
 struct ps2_gpio_data {
 	struct device *dev;
 	struct serio *serio;
@@ -52,6 +76,8 @@ struct ps2_gpio_data {
 	struct gpio_desc *gpio_data;
 	bool write_enable;
 	int irq;
+	ktime_t t_irq_now;
+	ktime_t t_irq_last;
 	struct ps2_gpio_data_rx {
 		unsigned char cnt;
 		unsigned char byte;
@@ -59,6 +85,8 @@ struct ps2_gpio_data {
 	struct ps2_gpio_data_tx {
 		unsigned char cnt;
 		unsigned char byte;
+		ktime_t t_xfer_start;
+		ktime_t t_xfer_end;
 		struct completion complete;
 		struct mutex mutex;
 		struct delayed_work work;
@@ -69,6 +97,9 @@ static int ps2_gpio_open(struct serio *serio)
 {
 	struct ps2_gpio_data *drvdata = serio->port_data;
 
+	drvdata->t_irq_last = 0;
+	drvdata->tx.t_xfer_end = 0;
+
 	enable_irq(drvdata->irq);
 	return 0;
 }
@@ -130,6 +161,7 @@ static void ps2_gpio_tx_work_fn(struct work_struct *work)
 	struct delayed_work *dwork = to_delayed_work(work);
 	struct ps2_gpio_data *drvdata = to_ps2_gpio_data(dwork);
 
+	drvdata->tx.t_xfer_start = ktime_get();
 	enable_irq(drvdata->irq);
 	gpiod_direction_output(drvdata->gpio_data, 0);
 	gpiod_direction_input(drvdata->gpio_clk);
@@ -140,20 +172,30 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 	unsigned char byte, cnt;
 	int data;
 	int rxflags = 0;
-	static unsigned long old_jiffies;
+	s64 us_delta;
 
 	byte = drvdata->rx.byte;
 	cnt = drvdata->rx.cnt;
 
-	if (old_jiffies == 0)
-		old_jiffies = jiffies;
+	drvdata->t_irq_now = ktime_get();
 
-	if ((jiffies - old_jiffies) > usecs_to_jiffies(100)) {
+	/* We need to consider spurious interrupts happening right after a TX xfer
+	 * finished.
+	 */
+	us_delta = ktime_us_delta(drvdata->t_irq_now, drvdata->tx.t_xfer_end);
+	if (unlikely(us_delta < PS2_IRQ_MIN_INTERVAL_US))
+		goto end;
+
+	us_delta = ktime_us_delta(drvdata->t_irq_now, drvdata->t_irq_last);
+	if (us_delta > PS2_IRQ_MAX_INTERVAL_US && cnt) {
 		dev_err(drvdata->dev,
 			"RX: timeout, probably we missed an interrupt\n");
 		goto err;
+	} else if (unlikely(us_delta < PS2_IRQ_MIN_INTERVAL_US)) {
+		/* Ignore spurious IRQs. */
+		goto end;
 	}
-	old_jiffies = jiffies;
+	drvdata->t_irq_last = drvdata->t_irq_now;
 
 	data = gpiod_get_value(drvdata->gpio_data);
 	if (unlikely(data < 0)) {
@@ -217,7 +259,7 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 			goto err;
 		}
 		cnt = byte = 0;
-		old_jiffies = 0;
+
 		goto end; /* success */
 	default:
 		dev_err(drvdata->dev, "RX: got out of sync with the device\n");
@@ -229,7 +271,6 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 
 err:
 	cnt = byte = 0;
-	old_jiffies = 0;
 	__ps2_gpio_write(drvdata->serio, PS2_CMD_RESEND);
 end:
 	drvdata->rx.cnt = cnt;
@@ -241,20 +282,32 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 {
 	unsigned char byte, cnt;
 	int data;
-	static unsigned long old_jiffies;
+	s64 us_delta;
 
 	cnt = drvdata->tx.cnt;
 	byte = drvdata->tx.byte;
 
-	if (old_jiffies == 0)
-		old_jiffies = jiffies;
+	drvdata->t_irq_now = ktime_get();
+
+	/* There might be pending IRQs since we disabled IRQs in __ps2_gpio_write().
+	 * We can expect at least one clock period until the device generates the
+	 * first falling edge after releasing the clock line.
+	 */
+	us_delta = ktime_us_delta(drvdata->t_irq_now,
+				  drvdata->tx.t_xfer_start);
+	if (unlikely(us_delta < PS2_CLK_MIN_INTERVAL_US))
+		goto end;
 
-	if ((jiffies - old_jiffies) > usecs_to_jiffies(100)) {
+	us_delta = ktime_us_delta(drvdata->t_irq_now, drvdata->t_irq_last);
+	if (us_delta > PS2_IRQ_MAX_INTERVAL_US && cnt > 1) {
 		dev_err(drvdata->dev,
 			"TX: timeout, probably we missed an interrupt\n");
 		goto err;
+	} else if (unlikely(us_delta < PS2_IRQ_MIN_INTERVAL_US)) {
+		/* Ignore spurious IRQs. */
+		goto end;
 	}
-	old_jiffies = jiffies;
+	drvdata->t_irq_last = drvdata->t_irq_now;
 
 	switch (cnt) {
 	case PS2_START_BIT:
@@ -295,11 +348,11 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 			goto err;
 		}
 
+		drvdata->tx.t_xfer_end = ktime_get();
 		drvdata->mode = PS2_MODE_RX;
 		complete(&drvdata->tx.complete);
 
 		cnt = 1;
-		old_jiffies = 0;
 		goto end; /* success */
 	default:
 		/* Probably we missed the stop bit. Therefore we release data
@@ -315,7 +368,6 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 
 err:
 	cnt = 1;
-	old_jiffies = 0;
 	gpiod_direction_input(drvdata->gpio_data);
 	__ps2_gpio_write(drvdata->serio, drvdata->tx.byte);
 end:
-- 
2.35.1

[PATCH 3/4] input: ps2-gpio: remove tx timeout from ps2_gpio_irq_tx()

[Danilo Krummrich]

Actually, there's no extra clock pulse to wait for.

The assumption of an extra clock pulse was mistakenly derived from the
fact that by the time this driver was introduced the GPIO controller of
the test machine (bcm2835) generated spurious interrupts.

Since now spurious interrupts are handled properly this can and must be
removed in order to make TX xfers work properly.

While at it, remove duplicate gpiod_direction_input(). The data gpio
must already be configured to act as input when receiving the ACK bit.

This patch is tested with the original hardware (peripherals and board)
the driver was developed on.

Signed-off-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
---
 drivers/input/serio/ps2-gpio.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/drivers/input/serio/ps2-gpio.c b/drivers/input/serio/ps2-gpio.c
index 246a583986e9..f47a967f7521 100644
--- a/drivers/input/serio/ps2-gpio.c
+++ b/drivers/input/serio/ps2-gpio.c
@@ -37,8 +37,7 @@
 #define PS2_DATA_BIT7		8
 #define PS2_PARITY_BIT		9
 #define PS2_STOP_BIT		10
-#define PS2_TX_TIMEOUT		11
-#define PS2_ACK_BIT		12
+#define PS2_ACK_BIT		11
 
 #define PS2_DEV_RET_ACK		0xfa
 #define PS2_DEV_RET_NACK	0xfe
@@ -335,13 +334,7 @@ static irqreturn_t ps2_gpio_irq_tx(struct ps2_gpio_data *drvdata)
 		/* release data line to generate stop bit */
 		gpiod_direction_input(drvdata->gpio_data);
 		break;
-	case PS2_TX_TIMEOUT:
-		/* Devices generate one extra clock pulse before sending the
-		 * acknowledgment.
-		 */
-		break;
 	case PS2_ACK_BIT:
-		gpiod_direction_input(drvdata->gpio_data);
 		data = gpiod_get_value(drvdata->gpio_data);
 		if (data) {
 			dev_warn(drvdata->dev, "TX: received NACK, retry\n");
-- 
2.35.1

[PATCH 4/4] input: ps2-gpio: don't send rx data before the stop bit

[Danilo Krummrich]

Sending the data before processing the stop bit from the device already
saves the data of the current xfer in case the stop bit is missed.

However, when TX xfers are enabled this introduces a race condition when
a peripheral driver using the bus immediately requests a TX xfer from IRQ
context.

Therefore the data must be send after receiving the stop bit, although
it is possible the data is lost when missing the stop bit.

Signed-off-by: Danilo Krummrich <danilokrummrich@dk-develop.de>
---
 drivers/input/serio/ps2-gpio.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/drivers/input/serio/ps2-gpio.c b/drivers/input/serio/ps2-gpio.c
index f47a967f7521..17091b137744 100644
--- a/drivers/input/serio/ps2-gpio.c
+++ b/drivers/input/serio/ps2-gpio.c
@@ -231,6 +231,13 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 			if (!drvdata->write_enable)
 				goto err;
 		}
+		break;
+	case PS2_STOP_BIT:
+		/* stop bit should be high */
+		if (unlikely(!data)) {
+			dev_err(drvdata->dev, "RX: stop bit should be high\n");
+			goto err;
+		}
 
 		/* Do not send spurious ACK's and NACK's when write fn is
 		 * not provided.
@@ -242,21 +249,9 @@ static irqreturn_t ps2_gpio_irq_rx(struct ps2_gpio_data *drvdata)
 				break;
 		}
 
-		/* Let's send the data without waiting for the stop bit to be
-		 * sent. It may happen that we miss the stop bit. When this
-		 * happens we have no way to recover from this, certainly
-		 * missing the parity bit would be recognized when processing
-		 * the stop bit. When missing both, data is lost.
-		 */
 		serio_interrupt(drvdata->serio, byte, rxflags);
 		dev_dbg(drvdata->dev, "RX: sending byte 0x%x\n", byte);
-		break;
-	case PS2_STOP_BIT:
-		/* stop bit should be high */
-		if (unlikely(!data)) {
-			dev_err(drvdata->dev, "RX: stop bit should be high\n");
-			goto err;
-		}
+
 		cnt = byte = 0;
 
 		goto end; /* success */
-- 
2.35.1

Re: ps2-gpio: use ktime for IRQ timekeeping

[Dmitry Torokhov]

On Tue, Feb 15, 2022 at 05:02:04PM +0100, Danilo Krummrich wrote:
> Changes since v1
> ================
>   - add patch to refactor struct ps2_gpio_data for clear separation between
>     RX and TX
>   - make all variables for IRQ timekeeping per-port and initialize them in
>     ps2_gpio_open()
> 
> This patch series implements the usage of ktime for IRQ timekeeping to
> overcome:
> 
> (1) The resolution limitations of jiffies.
> (2) Potential spurious IRQs generated by gpio controllers.
> 
> Besides that, based on the newly implemented timekeeping, it fixes a wrongly
> suspected extra clock cycle for TX transfers and a race condition when
> starting an immediate TX transfer based on data received from an RX transfer.
> 
> Danilo Krummrich (4):
>       input: ps2-gpio: refactor struct ps2_gpio_data
>       input: ps2-gpio: use ktime for IRQ timekeeping
>       input: ps2-gpio: remove tx timeout from ps2_gpio_irq_tx()
>       input: ps2-gpio: don't send rx data before the stop bit
> 
>  drivers/input/serio/ps2-gpio.c | 180 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------------------------------------------
>  1 file changed, 116 insertions(+), 64 deletions(-)

Applied the lot, thank you.

-- 
Dmitry