Re: [PATCH v2 4/5] iio:magnetometer:ak8975: mounting matrix support

Gregor Boirie <gregor.boirie@xxxxxxxxxx> · Wed, 9 Mar 2016 11:56:13 +0100

On 03/05/2016 04:36 PM, Jonathan Cameron wrote:
On 03/03/16 10:44, Gregor Boirie wrote:
Expose a rotation matrix to indicate userspace the chip placement with
respect to the overall hardware system. This is needed to adjust
coordinates sampled from a magnetometer chip when its position deviates
from the main hardware system.

Final coordinates computation is delegated to userspace since:
* computation may involve floating point arithmetics ;
* it allows an application to combine adjustments with arbitrary
   transformations.

This 3 dimentional space rotation matrix is expressed as 3x3 array of
strings to support floating point numbers. It may be retrieved from a
"in_magn_matrix" sysfs attribute file. It is declared into ak8975 DTS
entry as a "matrix" property.

Signed-off-by: Gregor Boirie <gregor.boirie@xxxxxxxxxx>
This one definitely wants to go to the device tree list. I'd also like a few
IIO related comments as well. I was sure we'd been through this before and
ended up with some sort of solution, but now I can't find it... Oh well.
I took inv_mpu_core as an example. However matrices are exposed as integers
and we really need floating point support.
A search on the device-tree mailing list archive does not show up with a
solution to encode floating point numbers into DTS either.


Definitely needs ABI docs in Documentation/ABI/testing/sysfs-bus-iio
as well. We need the reference directions to be well specified as
well - how do we define the main hardware orientation?  on yours presumably
the usual approach of using the screen is irrelevant so that definition
could be a little 'interesting' to write down?
The standard orientation for us is defined with respect to the drone's head
and the propellers plane, which does not suit the nominal use cases 
indeed...
Although it really deserves some more detailed documentation, I wonder 
if the
question is relevant.
Given that the DTS is board specific (correct me if I'm wrong), why not
consider the reference orientation as board specific as well (and make the
whole thing implicit) ?


I'm also unsure of whether the right thing to do in the 'not provided'
cases is to not export the setting, or to assume that the chip is at some
'standard' orientation and provide that?
As a self-explanatory interface, I suppose sysfs convention would expect 
to expose
this even in the case of an identity matrix.
However, userspace would need to either:
* systematically apply rotation matrix ;
* or detect identity matrix and skip rotation which may require an a-priori
  knowledge of reference orientation (breaking the statement mentioned 
above).

Personally, I'd prefer not exporting the property at all when aligned with
the main hardware orientation : this makes application simpler and 
allows for
better performances.

What do you think ?
Grégor.


Jonathan

---
  .../bindings/iio/magnetometer/ak8975.txt           | 10 +++++
  drivers/iio/magnetometer/ak8975.c                  | 46 +++++++++++++++++++++-
  2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt b/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
index 34a3206..f936f86 100644
--- a/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
+++ b/Documentation/devicetree/bindings/iio/magnetometer/ak8975.txt
@@ -9,6 +9,7 @@ Optional properties:
  
    - gpios : should be device tree identifier of the magnetometer DRDY pin
    - vdd-supply: an optional regulator that needs to be on to provide VDD
+  - matrix: an optional 3x3 mounting rotation matrix
  
  Example:
  
@@ -17,4 +18,13 @@ ak8975@0c {
          reg = <0x0c>;
          gpios = <&gpj0 7 0>;
          vdd-supply = <&ldo_3v3_gnss>;
+        matrix = "-0.984807753012208",  /* x0 */
+                 "0",                   /* y0 */
+                 "-0.173648177666930",  /* z0 */
+                 "0",                   /* x1 */
+                 "-1",                  /* y1 */
+                 "0",                   /* z1 */
+                 "-0.173648177666930",  /* x2 */
+                 "0",                   /* y2 */
+                 "0.984807753012208";   /* z2 */
  };
diff --git a/drivers/iio/magnetometer/ak8975.c b/drivers/iio/magnetometer/ak8975.c
index 72c03d9..95c68952 100644
--- a/drivers/iio/magnetometer/ak8975.c
+++ b/drivers/iio/magnetometer/ak8975.c
@@ -370,6 +370,7 @@ struct ak8975_data {
  	wait_queue_head_t	data_ready_queue;
  	unsigned long		flags;
  	u8			cntl_cache;
+	const char		*matrix[9];
  };
  
  /*
@@ -697,6 +698,28 @@ static int ak8975_read_raw(struct iio_dev *indio_dev,
  	return -EINVAL;
  }
  
+static ssize_t ak8975_show_matrix(struct device *dev,
+				  struct device_attribute *attr,
+				  char *buf)
+{
+	const struct ak8975_data *data = iio_priv(dev_to_iio_dev(dev));
+	const char * const *m = data->matrix;
+
+	return sprintf(buf, "%s, %s, %s; %s, %s, %s; %s, %s, %s\n",
+		       m[0], m[1], m[2], m[3], m[4], m[5], m[6], m[7], m[8]);
+}
+
+static IIO_DEVICE_ATTR(in_magn_matrix, S_IRUGO, ak8975_show_matrix, NULL, -1);
+
+static struct attribute *ak8975_attrs[] = {
+	&iio_dev_attr_in_magn_matrix.dev_attr.attr,
+	NULL
+};
+
+static const struct attribute_group ak8975_attrs_group = {
+	.attrs = ak8975_attrs
+};
+
  #define AK8975_CHANNEL(axis, index)					\
  	{								\
  		.type = IIO_MAGN,					\
@@ -716,6 +739,12 @@ static const struct iio_info ak8975_info = {
  	.driver_module = THIS_MODULE,
  };
  
+static const struct iio_info ak8975_matrix_info = {
+	.read_raw = &ak8975_read_raw,
+	.attrs = &ak8975_attrs_group,
+	.driver_module = THIS_MODULE,
+};
+
  static const struct acpi_device_id ak_acpi_match[] = {
  	{"AK8975", AK8975},
  	{"AK8963", AK8963},
@@ -785,6 +814,22 @@ static int ak8975_probe(struct i2c_client *client,
  	data->eoc_gpio = eoc_gpio;
  	data->eoc_irq = 0;
  
+	/*
+	 * Rotation matrix is expressed as an array of 3x3 strings to be able
+	 * to represent floating point numbers.
+	 */
+	err = of_property_read_string_array(client->dev.of_node, "matrix",
+					    data->matrix,
+					    ARRAY_SIZE(data->matrix));
+	if (err == ARRAY_SIZE(data->matrix))
+		indio_dev->info = &ak8975_matrix_info;
+	else if (err == -EINVAL)
+		indio_dev->info = &ak8975_info;
+	else if (err >= 0)
+		return -EINVAL;
+	else
+		return err;
+
  	/* id will be NULL when enumerated via ACPI */
  	if (id) {
  		chipset = (enum asahi_compass_chipset)(id->driver_data);
@@ -822,7 +867,6 @@ static int ak8975_probe(struct i2c_client *client,
  	indio_dev->dev.parent = &client->dev;
  	indio_dev->channels = ak8975_channels;
  	indio_dev->num_channels = ARRAY_SIZE(ak8975_channels);
-	indio_dev->info = &ak8975_info;
  	indio_dev->modes = INDIO_DIRECT_MODE;
  	indio_dev->name = name;
  	return devm_iio_device_register(&client->dev, indio_dev);


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