From: Andrea Merello andrea.merello@gmail.com
[ Upstream commit 3b71f6b59508b1c9befcb43de434866aafc76520 ]
Each time we need to read a sample (from the sysfs interface, since the driver supports only it) the driver writes the configuration register with the proper settings needed to perform the said read, then it runs another xfer to actually read the resulting value. Most notably the configuration register is updated to set the ADC internal MUX depending by which channel the read targets.
Unfortunately this seems not enough to ensure correct operation because the ADC works in a pipelined-like fashion and the new configuration isn't applied in time.
The ADC alternates two phases: acquisition and conversion. During the acquisition phase the ADC samples the analog signal in an internal capacitor; in the conversion phase the ADC performs the actual analog to digital conversion of the stored voltage. Note that of course the MUX needs to be set to the proper channel when the acquisition phase is performed.
Once the conversion phase has been completed, the device automatically switches back to a new acquisition; on the other hand the device switches from acquisition to conversion on the rising edge of SPI cs signal (that is when the xfer finishes).
Only after both two phases have been completed (with the proper settings already written in the configuration register since the beginning) it is possible to read the outcome from SPI bus.
With the current driver implementation, we end up in the following situation:
_______ 1st xfer ____________ 2nd xfer ___________________ SPI cs.. _________/ _________/ SPI rd.. idle |(val N-2)+ idle | val N-1 + idle ... SPI wr.. idle | cfg N + idle | (X) + idle ... ------------------------ + -------------------- + ------------------ AD .. acq N-1 + cnv N-1 | acq N + cnv N | acq N+1
As shown in the diagram above, the value we read in the Nth read belongs to configuration setting N-1.
In case the configuration is not changed (config[N] == config[N-1]), then we still get correct data, but in case the configuration changes (i.e. switching the MUX on another channel), we get wrong data (data from the previously selected channel).
This patch fixes this by performing one more "dummy" transfer in order to ending up in reading the data when it's really ready, as per the following timing diagram.
_______ 1st xfer ____________ 2nd xfer ___________ 3rd xfer ___ SPI cs.. _________/ _________/ _________/ SPI rd.. idle |(val N-2)+ idle |(val N-1)+ idle | val N + .. SPI wr.. idle | cfg N + idle | (X) + idle | (X) + .. ------------------------ + -------------------- + ------------------- + -- AD .. acq N-1 + cnv N-1 | acq N + cnv N | acq N+1 | ..
NOTE: in the latter case (cfg changes), the acquisition phase for the value to be read begins after the 1st xfer, that is after the read request has been issued on sysfs. On the other hand, if the cfg doesn't change, then we can refer to the fist diagram assuming N == (N - 1); the acquisition phase _begins_ before the 1st xfer (potentially a lot of time before the read has been issued via sysfs, but it _ends_ after the 1st xfer, that is _after_ the read has started. This should guarantee a reasonably fresh data, which value represents the voltage that the sampled signal has after the read start or maybe just around it.
Signed-off-by: Andrea Merello andrea.merello@gmail.com Reviewed-by: Charles-Antoine Couret charles-antoine.couret@essensium.com Cc: Stable@vger.kernel.org Signed-off-by: Jonathan Cameron Jonathan.Cameron@huawei.com Signed-off-by: Sasha Levin sashal@kernel.org --- drivers/iio/adc/ad7949.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-)
diff --git a/drivers/iio/adc/ad7949.c b/drivers/iio/adc/ad7949.c index 518044c31a73b..6b51bfcad0d04 100644 --- a/drivers/iio/adc/ad7949.c +++ b/drivers/iio/adc/ad7949.c @@ -89,6 +89,7 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val, unsigned int channel) { int ret; + int i; int bits_per_word = ad7949_adc->resolution; int mask = GENMASK(ad7949_adc->resolution, 0); struct spi_message msg; @@ -100,12 +101,23 @@ static int ad7949_spi_read_channel(struct ad7949_adc_chip *ad7949_adc, int *val, }, };
- ret = ad7949_spi_write_cfg(ad7949_adc, - channel << AD7949_OFFSET_CHANNEL_SEL, - AD7949_MASK_CHANNEL_SEL); - if (ret) - return ret; + /* + * 1: write CFG for sample N and read old data (sample N-2) + * 2: if CFG was not changed since sample N-1 then we'll get good data + * at the next xfer, so we bail out now, otherwise we write something + * and we read garbage (sample N-1 configuration). + */ + for (i = 0; i < 2; i++) { + ret = ad7949_spi_write_cfg(ad7949_adc, + channel << AD7949_OFFSET_CHANNEL_SEL, + AD7949_MASK_CHANNEL_SEL); + if (ret) + return ret; + if (channel == ad7949_adc->current_channel) + break; + }
+ /* 3: write something and read actual data */ ad7949_adc->buffer = 0; spi_message_init_with_transfers(&msg, tx, 1); ret = spi_sync(ad7949_adc->spi, &msg);