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Andrea Merello af5b2e18ae iio: ad7949: fix channels mixups 
[ Upstream commit 3b71f6b595 ]

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>
 		2019-12-17 19:56:45 +01:00
Documentation USB: documentation: flags on usb-storage versus UAS 2019-12-17 19:55:32 +01:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch powerpc/xive: Skip ioremap() of ESB pages for LSI interrupts 2019-12-17 19:56:42 +01:00
block blk-mq: avoid sysfs buffer overflow with too many CPU cores 2019-12-17 19:56:14 +01:00
certs PKCS#7: Refactor verify_pkcs7_signature() 2019-08-05 18:40:18 -04:00
crypto crypto: user - fix memory leak in crypto_reportstat 2019-12-13 08:43:11 +01:00
drivers iio: ad7949: fix channels mixups 2019-12-17 19:56:45 +01:00
fs reiserfs: fix extended attributes on the root directory 2019-12-17 19:56:44 +01:00
include quota: Check that quota is not dirty before release 2019-12-17 19:56:43 +01:00
init Merge branch 'next-lockdown' of git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security 2019-09-28 08:14:15 -07:00
ipc ipc/sem.c: convert to use built-in RCU list checking 2019-09-25 17:51:41 -07:00
kernel cgroup: pids: use atomic64_t for pids->limit 2019-12-17 19:56:15 +01:00
lib lib: raid6: fix awk build warnings 2019-12-17 19:56:09 +01:00
mm mm/ksm.c: don't WARN if page is still mapped in remove_stable_node() 2019-11-22 09:11:18 -08:00
net rfkill: allocate static minor 2019-12-13 08:43:18 +01:00
samples samples/bpf: fix build by setting HAVE_ATTR_TEST to zero 2019-10-31 21:39:15 +01:00
scripts arm64 fix for -rc8 / final 2019-11-15 09:14:23 -08:00
security efi/efi_test: Lock down /dev/efi_test and require CAP_SYS_ADMIN 2019-10-31 09:40:21 +01:00
sound ALSA: hda/realtek - Line-out jack doesn't work on a Dell AIO 2019-12-17 19:56:28 +01:00
tools seccomp: avoid overflow in implicit constant conversion 2019-12-17 19:56:43 +01:00
usr kbuild: update compile-test header list for v5.4-rc2 2019-10-05 15:29:49 +09:00
virt KVM: arm/arm64: vgic: Don't rely on the wrong pending table 2019-12-13 08:43:00 +01:00
.clang-format clang-format: Update with the latest for_each macro list 2019-08-31 10:00:51 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Modules updates for v5.4 2019-09-22 10:34:46 -07:00
.mailmap ARM: SoC fixes 2019-11-10 13:41:59 -08:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer 2019-10-10 08:12:51 -07:00
Kbuild kbuild: do not descend to ./Kbuild when cleaning 2019-08-21 21:03:58 +09:00
Kconfig docs: kbuild: convert docs to ReST and rename to *.rst 2019-06-14 14:21:21 -06:00
MAINTAINERS Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2019-11-22 14:28:14 -08:00
Makefile Linux 5.4.3 2019-12-13 08:43:32 +01:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.