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Subject[RFC 0/7] iio: inv_mpu6050: Support i2c master and external readings
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This series attempts to implement support for external readings in i2c master
mode. I don't expect this to go in this form but I wanted to present a
functional version in order to start the discussion earlier.


The I2C master support is useful even without external readings. For example in
SPI mode this is the only way to contact auxiliary devices. Differences from
the previous version:
- Require an explicit "inv,i2c-aux-master" bool property. Supporting dynamic
switches between bypass/master mode is extremely difficult and not really
worthwhile. Switching to bypass mode requires explicitly disabling all i2c
master functionality, including external readings! After Peter Rosin's mux
cleanup patches go in I would like to disable the mux entirely when in master
mode.
- Describe i2c clients behind i2c@1. Maybe it should be i2c@0 like mux mode?
- Validate parameters and return an error on unsupported ops (like read_word)
- Wait for SLV4_DONE even on writes!
- Fix issues when other parts of the driver write to the same registers.

My initial idea was to use regcache and regmap_update_bits to handle config
bits like in PATCH 2. It turn out that the device has several registers with a
combination of volatile "command bits" (writing triggers an action) and "config
bits" (value must normally be preserved). This requires manually storing the
state of those config bits in "chip_config". I could try to make do without
regcache support, even though it offers some advantages.


Support for external sensors is rather nasty. What it does is dynamically copy
iio_chan_spec from the slave device and register it for itself (with an updated
scan index). Then it forwards everything but IIO_CHAN_INFO_RAW to the other
driver while serving raw value requests from the MPU's own registers.

The original device is still registered with linux as a normal driver and works
normally and you can poke at it to configure stuff like sample rates and
scaling factors. You can read the same samples from the slaved device, just
slower. For example:
cat /sys/bus/iio/devices/iio:device1/in_magn_*_raw
will be slower than:
cat /sys/bus/iio/devices/iio:device0/in_magn_*_raw
In the first case values are read through SLV4, byte-by-byte. In the second
they are served directly from EXT_SENS_DATA_* registers.

In theory the ak8975 inside mpu9150 could be supported as a slave device but
that requires further work. Unlike the hmc5883l I've been testing with that
sensor does not update automatically by default. This means that the mpu6050
buffer code must call a function from that driver to "enable automatic updates"
somehow. For example this could be implemented as an additional bufferop?

So far this works surprisingly well without any changes in the iio core or
drivers for aux devices. But perhaps "external channels" should be listed in
struct iio_dev and a special marking should be done for iio_chan_spec's which
can accessed this way.

External/slaved channels are limited to 16bits because it would otherwise be
difficult to comply with iio alignment requirements. Support for other channel
sizes should be implemented separately. There should be a separate discussion
for how to properly support driver-specified channel offsets instead of
implicit iio rules.

Patches 1,2,3,4 and 6 are required cleanups/fixed to make the rest work. They
could go in separately.

Crestez Dan Leonard (7):
iio: inv_mpu6050: Do burst reads using spi/i2c directly
iio: inv_mpu6050: Initial regcache support
iio: inv_mpu6050: Only toggle DATA_RDY_EN in inv_reset_fifo
iio: inv_mpu6050: Cache non-volatile bits of user_ctrl
iio: inv_mpu6050: Add support for auxiliary I2C master
iio: inv_mpu6050: Check channel configuration on preenable
iio: inv_mpu6050: Add support for external sensors

.../devicetree/bindings/iio/imu/inv_mpu6050.txt | 96 ++-
drivers/iio/imu/inv_mpu6050/inv_mpu_core.c | 708 ++++++++++++++++++++-
drivers/iio/imu/inv_mpu6050/inv_mpu_i2c.c | 5 -
drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h | 116 +++-
drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c | 134 +++-
drivers/iio/imu/inv_mpu6050/inv_mpu_spi.c | 5 -
drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c | 15 +-
7 files changed, 1029 insertions(+), 50 deletions(-)

--
2.5.5

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