Re: [RFC] hwmon: ina2xx: allow for actual measurement bandwidth above 160 Hz

From: Guenter Roeck
Date: Mon Oct 19 2015 - 21:32:15 EST

On 10/19/2015 09:21 AM, Marc Titinger wrote:
With the current implementation, the driver will prevent a readout at a
pace faster than the default conversion time (2ms) times the averaging
setting, min AVG being 1:1.

Any sysfs "show" read access from the client app faster than 500 Hz will be
'cached' by the driver, but actually since do_update reads all 8 registers,
the best achievable measurement rate is roughly 8*800 us (for the time
spent in i2c-core) i.e. <= 156Hz with Beagle Bone Black.

This change set uses a register mask to allow for the readout of a single
i2c register at a time. Furthermore, performing subsequent reads on the
same register will make use of the ability of the i2c chip to retain the
last reg offset, hence use a shorter i2c message (roughly 400us instead of
800us spent in i2c-core.c).

The best readout rate for a single measurement is now around 2kHz. And for
four measurements around (1/(4*800us) = 312 Hz. Since for any readout rate
faster than 160 Hz the interval is set by the i2c transactions completion,
the 'last-update' anti-flooding code will not have a limiting effect in
practice. Hence I also remove the elapsed time checking in the hwmon driver
for ina2xx.

To summarize, the patch provides a max bandwidth improvement with hwmon
client apps from ~160 Hz to ~320 Hz, and better in single-channel
polling mode.


I really dislike that complexity. Maybe we should drop caching entirely
in this driver ? Maybe even convert it to use regmap ?

Guenter

Signed-off-by: Marc Titinger <mtitinger@xxxxxxxxxxxx>
---
drivers/hwmon/ina2xx.c | 90 +++++++++++++++++++++++++++++++++++++++-----------
1 file changed, 71 insertions(+), 19 deletions(-)

diff --git a/drivers/hwmon/ina2xx.c b/drivers/hwmon/ina2xx.c
index 4d28150..ce3a2ee 100644
--- a/drivers/hwmon/ina2xx.c
+++ b/drivers/hwmon/ina2xx.c
@@ -48,6 +48,8 @@
#define INA2XX_CURRENT 0x04 /* readonly */
#define INA2XX_CALIBRATION 0x05

+#define BITPOS_TO_MASK(x) (1L << x)
+
/* INA226 register definitions */
#define INA226_MASK_ENABLE 0x06
#define INA226_ALERT_LIMIT 0x07
@@ -105,9 +107,14 @@ struct ina2xx_data {

struct mutex update_lock;
bool valid;
- unsigned long last_updated;
int update_interval; /* in jiffies */

+ /* Last read register (slave address already set
+ * reading out from this same register repeatedly will
+ * be significantly faster!
+ */
+ int last_reg;
+
int kind;
const struct attribute_group *groups[INA2XX_MAX_ATTRIBUTE_GROUPS];
u16 regs[INA2XX_MAX_REGISTERS];
@@ -203,21 +210,63 @@ static int ina2xx_init(struct ina2xx_data *data)
return ina2xx_calibrate(data);
}

-static int ina2xx_do_update(struct device *dev)
+/*
+ * Most I2c chips will allow reading from the current register pointer
+ * w/o setting the register offset again.
+ */
+static inline s32 __i2c_read_same_word(const struct i2c_client *client)
+{
+ unsigned char msgbuf[2];
+
+ struct i2c_msg msg = {
+ .addr = client->addr,
+ .flags = client->flags | I2C_M_RD,
+ .len = 2,
+ .buf = msgbuf,
+ };
+
+ int status = i2c_transfer(client->adapter, &msg, 1);
+
+ return (status < 0) ? status : (msgbuf[1]|(msgbuf[0]<<8));
+}
+
+static int ina2xx_do_update(struct device *dev, int reg_mask)
{
struct ina2xx_data *data = dev_get_drvdata(dev);
struct i2c_client *client = data->client;
- int i, rv, retry;
+ int i = 0, rv, retry;

dev_dbg(&client->dev, "Starting ina2xx update\n");

for (retry = 5; retry; retry--) {
- /* Read all registers */
- for (i = 0; i < data->config->registers; i++) {
- rv = i2c_smbus_read_word_swapped(client, i);
+
+ /* Try to issue a shorter i2c message */
+ if (reg_mask & (1 << data->last_reg)) {
+ rv = __i2c_read_same_word(client);
if (rv < 0)
return rv;
- data->regs[i] = rv;
+
+ reg_mask &= ~(1 << data->last_reg);
+ data->regs[data->last_reg] = rv;
+
+ dev_dbg(&client->dev, "%d, rv = %x, (last_reg)\n",
+ data->last_reg,
+ data->regs[data->last_reg]);
+ }
+
+ /* Check for remaining registers in mask. */
+ while (reg_mask && i < data->config->registers) {
+ if (reg_mask & (1L << i)) {
+ rv = i2c_smbus_read_word_swapped(client, i);
+ if (rv < 0)
+ return rv;
+ data->regs[i] = rv;
+ data->last_reg = i;
+
+ dev_dbg(&client->dev, "%d, rv = %x\n", i,
+ data->regs[i]);
+ }
+ i++;
}

/*
@@ -240,8 +289,6 @@ static int ina2xx_do_update(struct device *dev)
msleep(INA2XX_MAX_DELAY);
continue;
}
-
- data->last_updated = jiffies;
data->valid = 1;

return 0;
@@ -256,22 +303,24 @@ static int ina2xx_do_update(struct device *dev)
return -ENODEV;
}

-static struct ina2xx_data *ina2xx_update_device(struct device *dev)
+static struct ina2xx_data *ina2xx_update_device(struct device *dev,
+ int reg_mask)
{
struct ina2xx_data *data = dev_get_drvdata(dev);
struct ina2xx_data *ret = data;
- unsigned long after;
int rv;

mutex_lock(&data->update_lock);

- after = data->last_updated + data->update_interval;
- if (time_after(jiffies, after) || !data->valid) {
- rv = ina2xx_do_update(dev);
- if (rv < 0)
- ret = ERR_PTR(rv);
+ if (!data->valid) {
+ reg_mask = 0xff; /* do all regs */
+ data->last_reg = 0xff;
}

+ rv = ina2xx_do_update(dev, reg_mask);
+ if (rv < 0)
+ ret = ERR_PTR(rv);
+
mutex_unlock(&data->update_lock);
return ret;
}
@@ -316,7 +365,8 @@ static ssize_t ina2xx_show_value(struct device *dev,
struct device_attribute *da, char *buf)
{
struct sensor_device_attribute *attr = to_sensor_dev_attr(da);
- struct ina2xx_data *data = ina2xx_update_device(dev);
+ struct ina2xx_data *data = ina2xx_update_device(dev,
+ BITPOS_TO_MASK(attr->index));

if (IS_ERR(data))
return PTR_ERR(data);
@@ -329,7 +379,8 @@ static ssize_t ina2xx_set_shunt(struct device *dev,
struct device_attribute *da,
const char *buf, size_t count)
{
- struct ina2xx_data *data = ina2xx_update_device(dev);
+ struct ina2xx_data *data = ina2xx_update_device(dev,
+ BITPOS_TO_MASK(INA2XX_CONFIG));
unsigned long val;
int status;

@@ -390,7 +441,8 @@ static ssize_t ina226_set_interval(struct device *dev,
static ssize_t ina226_show_interval(struct device *dev,
struct device_attribute *da, char *buf)
{
- struct ina2xx_data *data = ina2xx_update_device(dev);
+ struct ina2xx_data *data = ina2xx_update_device(dev,
+ BITPOS_TO_MASK(INA2XX_CONFIG));

if (IS_ERR(data))
return PTR_ERR(data);


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