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

[Marc Titinger]

On 20/10/2015 03:32, Guenter Roeck wrote:
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.


Hi Guenter,

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

I dropped caching completely with this patch. The only (mild!) 
complexity added is to keep track of the current register pointer
set in the chip, to issue a shorter i2c message.

Thanks for the hint re. regmap, is there a driver file you recommend I 
look into (because the i2c caps were close to in2xx) ?

Many thanks,
Marc.



Guenter

Signed-off-by: Marc Titinger <mtitin...@baylibre.com>
---
  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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