spi 3wire support for ST MEMs sensors

spi 3wire support for ST MEMs sensors

[Lorenzo Bianconi]

Hi all,

I started working on supporting spi-3wire mode in some STM MEMs
drivers and I am facing a tricky issue. Let's consider the driver for
LSM303AGR accel device
(http://www.st.com/resource/en/datasheet/lsm303agr.pdf). The sensor is
configured by default in spi 4 wire mode (BIT 0 in register 0x23) but
the package does not export the MISO line for accelerometer. The only
way to support spi is to enable 3-wire at bootstrap. I am wondering
how to figure out the correct register address since it is not
possible to read the device whoami. The only way that come up to my
mind is to add a lookup table in st_accel_spi.c based on
spi_device_id. Is there something better? Or should I just remove the
spi support for LSM303AGR? Thanks

Regards,
Lorenzo

-- 
UNIX is Sexy: who | grep -i blonde | talk; cd ~; wine; talk; touch;
unzip; touch; strip; gasp; finger; gasp; mount; fsck; more; yes; gasp;
umount; make clean; sleep

Re: spi 3wire support for ST MEMs sensors

[Jonathan Cameron]

On Fri, 9 Jun 2017 11:30:00 +0200
Lorenzo Bianconi <lorenzo.bianconi83@gmail.com> wrote:

> Hi all,
> 
> I started working on supporting spi-3wire mode in some STM MEMs
> drivers and I am facing a tricky issue. Let's consider the driver for
> LSM303AGR accel device
> (http://www.st.com/resource/en/datasheet/lsm303agr.pdf). The sensor is
> configured by default in spi 4 wire mode (BIT 0 in register 0x23) but
> the package does not export the MISO line for accelerometer. The only
> way to support spi is to enable 3-wire at bootstrap. I am wondering
> how to figure out the correct register address since it is not
> possible to read the device whoami. The only way that come up to my
> mind is to add a lookup table in st_accel_spi.c based on
> spi_device_id. Is there something better? Or should I just remove the
> spi support for LSM303AGR? Thanks
> 
> Regards,
> Lorenzo
> 

Hi Lorenzo,

Look up table is probably the best option when a device only supports
3 wire mode.

However, don't we need DT of ACPI to provide info on whether a 3 wire
bus is being used?  If so can we query it from the spi core?

Jonathan

Re: spi 3wire support for ST MEMs sensors

[Lorenzo Bianconi]

> On Fri, 9 Jun 2017 11:30:00 +0200
> Lorenzo Bianconi <lorenzo.bianconi83@gmail.com> wrote:
>
>> Hi all,
>>
>> I started working on supporting spi-3wire mode in some STM MEMs
>> drivers and I am facing a tricky issue. Let's consider the driver for
>> LSM303AGR accel device
>> (http://www.st.com/resource/en/datasheet/lsm303agr.pdf). The sensor is
>> configured by default in spi 4 wire mode (BIT 0 in register 0x23) but
>> the package does not export the MISO line for accelerometer. The only
>> way to support spi is to enable 3-wire at bootstrap. I am wondering
>> how to figure out the correct register address since it is not
>> possible to read the device whoami. The only way that come up to my
>> mind is to add a lookup table in st_accel_spi.c based on
>> spi_device_id. Is there something better? Or should I just remove the
>> spi support for LSM303AGR? Thanks
>>
>> Regards,
>> Lorenzo
>>
>
> Hi Lorenzo,
>

Hi Jonathan,

> Look up table is probably the best option when a device only supports
> 3 wire mode.
>
> However, don't we need DT of ACPI to provide info on whether a 3 wire
> bus is being used?  If so can we query it from the spi core?
>

Right, the user has to request the 3-wire mode using the standard
spi-3wire property.

> Jonathan

Regards,
Lorenzo

-- 
UNIX is Sexy: who | grep -i blonde | talk; cd ~; wine; talk; touch;
unzip; touch; strip; gasp; finger; gasp; mount; fsck; more; yes; gasp;
umount; make clean; sleep