[PATCH] iio: imu: inv_mpu6050: add comment about frequencies

[PATCH] iio: imu: inv_mpu6050: add comment about frequencies

[Martin Kelly]

Although the driver allows frequencies between 4 and 1000 Hz, only the
frequencies advertised in the available frequencies file are backed
properly by a low-pass filter to prevent aliasing, so it's best to use
them. Since this is not obvious to the user, add a comment explaining
what's going on.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
---
 drivers/iio/imu/inv_mpu6050/inv_mpu_core.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
index 43fba5f7532b..f9c0624505a2 100644
--- a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
+++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
@@ -798,7 +798,14 @@ static const struct iio_chan_spec inv_mpu_channels[] = {
 	INV_MPU6050_CHAN(IIO_ACCEL, IIO_MOD_Z, INV_MPU6050_SCAN_ACCL_Z),
 };
 
-/* constant IIO attribute */
+/*
+ * The user can choose any frequency between INV_MPU6050_MIN_FIFO_RATE and
+ * INV_MPU6050_MAX_FIFO_RATE, but only these frequencies are matched by the
+ * low-pass filter. Specifically, each of these sampling rates are about twice
+ * the bandwidth of a corresponding low-pass filter, which should eliminate
+ * aliasing following the Nyquist principle. By picking a frequency different
+ * from these, the user risks aliasing effects.
+ */
 static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("10 20 50 100 200 500");
 static IIO_CONST_ATTR(in_anglvel_scale_available,
 					  "0.000133090 0.000266181 0.000532362 0.001064724");
-- 
2.11.0

Re: [PATCH] iio: imu: inv_mpu6050: add comment about frequencies

[Jean-Baptiste Maneyrol]

Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>


From: Martin Kelly <mkelly@xevo.com>
Sent: Monday, May 21, 2018 20:41
To: linux-iio@vger.kernel.org
Cc: Jean-Baptiste Maneyrol; Jonathan Cameron; Martin Kelly
Subject: [PATCH] iio: imu: inv_mpu6050: add comment about frequencies

Although the driver allows frequencies between 4 and 1000 Hz, only the
frequencies advertised in the available frequencies file are backed
properly by a low-pass filter to prevent aliasing, so it's best to use
them. Since this is not obvious to the user, add a comment explaining
what's going on.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
---
=A0drivers/iio/imu/inv_mpu6050/inv_mpu_core.c | 9 ++++++++-
=A01 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c b/drivers/iio/imu/i=
nv_mpu6050/inv_mpu_core.c
index 43fba5f7532b..f9c0624505a2 100644
--- a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
+++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
@@ -798,7 +798,14 @@ static const struct iio_chan_spec inv_mpu_channels[] =
=3D {
=A0=A0=A0=A0=A0=A0=A0 INV_MPU6050_CHAN(IIO_ACCEL, IIO_MOD_Z, INV_MPU6050_SC=
AN_ACCL_Z),
=A0};

-/* constant IIO attribute */
+/*
+ * The user can choose any frequency between INV_MPU6050_MIN_FIFO_RATE and
+ * INV_MPU6050_MAX_FIFO_RATE, but only these frequencies are matched by th=
e
+ * low-pass filter. Specifically, each of these sampling rates are about t=
wice
+ * the bandwidth of a corresponding low-pass filter, which should eliminat=
e
+ * aliasing following the Nyquist principle. By picking a frequency differ=
ent
+ * from these, the user risks aliasing effects.
+ */
=A0static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("10 20 50 100 200 500");
=A0static IIO_CONST_ATTR(in_anglvel_scale_available,
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=
=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0 "0.000133090 0.000266181 0=
.000532362 0.001064724");
--
2.11.0

    =

Re: [PATCH] iio: imu: inv_mpu6050: add comment about frequencies

[Jonathan Cameron]

On Tue, 22 May 2018 12:55:23 +0000
Jean-Baptiste Maneyrol <JManeyrol@invensense.com> wrote:

> Acked-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
>=20
>=20
> From: Martin Kelly <mkelly@xevo.com>
> Sent: Monday, May 21, 2018 20:41
> To: linux-iio@vger.kernel.org
> Cc: Jean-Baptiste Maneyrol; Jonathan Cameron; Martin Kelly
> Subject: [PATCH] iio: imu: inv_mpu6050: add comment about frequencies
>=20
> Although the driver allows frequencies between 4 and 1000 Hz, only the
> frequencies advertised in the available frequencies file are backed
> properly by a low-pass filter to prevent aliasing, so it's best to use
> them. Since this is not obvious to the user, add a comment explaining
> what's going on.
>=20
> Signed-off-by: Martin Kelly <mkelly@xevo.com>

Applied to the togreg branch of iio.git and pushed out as testing for the
autobuilders to play with it.

Thanks,

Jonathan

> ---
> =C2=A0drivers/iio/imu/inv_mpu6050/inv_mpu_core.c | 9 ++++++++-
> =C2=A01 file changed, 8 insertions(+), 1 deletion(-)
>=20
> diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c b/drivers/iio/imu=
/inv_mpu6050/inv_mpu_core.c
> index 43fba5f7532b..f9c0624505a2 100644
> --- a/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
> +++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_core.c
> @@ -798,7 +798,14 @@ static const struct iio_chan_spec inv_mpu_channels[]=
 =3D {
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 INV_MPU6050_CHAN(IIO_ACCEL, II=
O_MOD_Z, INV_MPU6050_SCAN_ACCL_Z),
> =C2=A0};
>=20
> -/* constant IIO attribute */
> +/*
> + * The user can choose any frequency between INV_MPU6050_MIN_FIFO_RATE a=
nd
> + * INV_MPU6050_MAX_FIFO_RATE, but only these frequencies are matched by =
the
> + * low-pass filter. Specifically, each of these sampling rates are about=
 twice
> + * the bandwidth of a corresponding low-pass filter, which should elimin=
ate
> + * aliasing following the Nyquist principle. By picking a frequency diff=
erent
> + * from these, the user risks aliasing effects.
> + */
> =C2=A0static IIO_CONST_ATTR_SAMP_FREQ_AVAIL("10 20 50 100 200 500");
> =C2=A0static IIO_CONST_ATTR(in_anglvel_scale_available,
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=
=C2=A0=C2=A0=C2=A0=C2=A0 "0.000133090 0.000266181 0.000532362 0.001064724");
> --
> 2.11.0
>=20
>     --
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