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Louis Chauvet e21b5fc5b8 spi: atmel: Fix clock issue when using devices with different polarities 
[ Upstream commit fc70d643a2 ]

The current Atmel SPI controller driver (v2) behaves incorrectly when
using two SPI devices with different clock polarities and GPIO CS.

When switching from one device to another, the controller driver first
enables the CS and then applies whatever configuration suits the targeted
device (typically, the polarities). The side effect of such order is the
apparition of a spurious clock edge after enabling the CS when the clock
polarity needs to be inverted wrt. the previous configuration of the
controller.

This parasitic clock edge is problematic when the SPI device uses that edge
for internal processing, which is perfectly legitimate given that its CS
was asserted. Indeed, devices such as HVS8080 driven by driver gpio-sr in
the kernel are shift registers and will process this first clock edge to
perform a first register shift. In this case, the first bit gets lost and
the whole data block that will later be read by the kernel is all shifted
by one.

    Current behavior:
      The actual switching of the clock polarity only occurs after the CS
      when the controller sends the first message:

    CLK ------------\   /-\ /-\
                    |   | | | |    . . .
                    \---/ \-/ \
    CS  -----\
             |
             \------------------

             ^      ^   ^
             |      |   |
             |      |   Actual clock of the message sent
             |      |
             |      Change of clock polarity, which occurs with the first
             |      write to the bus. This edge occurs when the CS is
             |      already asserted, and can be interpreted as
             |      the first clock edge by the receiver.
             |
             GPIO CS toggle

This issue is specific to this controller because while the SPI core
performs the operations in the right order, the controller however does
not. In practice, the controller only applies the clock configuration right
before the first transmission.

So this is not a problem when using the controller's dedicated CS, as the
controller does things correctly, but it becomes a problem when you need to
change the clock polarity and use an external GPIO for the CS.

One possible approach to solve this problem is to send a dummy message
before actually activating the CS, so that the controller applies the clock
polarity beforehand.

New behavior:

CLK     ------\      /-\     /-\      /-\     /-\
              |      | | ... | |      | | ... | |
              \------/ \-   -/ \------/ \-   -/ \------

CS      -\/-----------------------\
         ||                       |
         \/                       \---------------------
         ^    ^       ^           ^    ^
         |    |       |           |    |
         |    |       |           |    Expected clock cycles when
         |    |       |           |    sending the message
         |    |       |           |
         |    |       |           Actual GPIO CS activation, occurs inside
         |    |       |           the driver
         |    |       |
         |    |       Dummy message, to trigger clock polarity
         |    |       reconfiguration. This message is not received and
         |    |       processed by the device because CS is low.
         |    |
         |    Change of clock polarity, forced by the dummy message. This
         |    time, the edge is not detected by the receiver.
         |
         This small spike in CS activation is due to the fact that the
         spi-core activates the CS gpio before calling the driver's
         set_cs callback, which deactivates this gpio again until the
         clock polarity is correct.

To avoid having to systematically send a dummy packet, the driver keeps
track of the clock's current polarity. In this way, it only sends the dummy
packet when necessary, ensuring that the clock will have the correct
polarity when the CS is toggled.

There could be two hardware problems with this patch:
1- Maybe the small CS activation peak can confuse SPI devices
2- If on a design, a single wire is used to select two devices depending
on its state, the dummy message may disturb them.

Fixes: 5ee36c9898 ("spi: atmel_spi update chipselect handling")
Cc:  <stable@vger.kernel.org>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Link: https://msgid.link/r/20231204154903.11607-1-louis.chauvet@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2024-01-05 15:18:38 +01:00
arch ARM: dts: Fix occasional boot hang for am3 usb 2024-01-05 15:18:37 +01:00
block blk-cgroup: bypass blkcg_deactivate_policy after destroying 2023-12-20 17:00:21 +01:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: pcrypt - Fix hungtask for PADATA_RESET 2023-11-28 17:06:58 +00:00
Documentation dt-bindings: nvmem: mxs-ocotp: Document fsl,ocotp 2024-01-01 12:39:04 +00:00
drivers spi: atmel: Fix clock issue when using devices with different polarities 2024-01-05 15:18:38 +01:00
fs nfsd: call nfsd_last_thread() before final nfsd_put() 2024-01-05 15:18:37 +01:00
include spi: Add APIs in spi core to set/get spi->chip_select and spi->cs_gpiod 2024-01-05 15:18:38 +01:00
init proc: sysctl: prevent aliased sysctls from getting passed to init 2023-11-28 17:07:08 +00:00
io_uring io_uring: fix mutex_unlock with unreferenced ctx 2023-12-13 18:39:18 +01:00
ipc ipc: fix memory leak in init_mqueue_fs() 2022-12-31 13:32:01 +01:00
kernel tracing / synthetic: Disable events after testing in synth_event_gen_test_init() 2024-01-01 12:39:07 +00:00
lib lib/vsprintf: Fix %pfwf when current node refcount == 0 2024-01-01 12:39:07 +00:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/damon/core: make damon_start() waits until kdamond_fn() starts 2024-01-01 12:39:08 +00:00
net wifi: nl80211: fix deadlock in nl80211_set_cqm_rssi (6.6.x) 2024-01-01 12:39:09 +00:00
rust rust: allocator: Prevent mis-aligned allocation 2023-08-11 12:08:18 +02:00
samples fprobe: Pass entry_data to handlers 2023-10-25 12:03:12 +02:00
scripts sign-file: Fix incorrect return values check 2023-12-20 17:00:19 +01:00
security keys, dns: Allow key types (eg. DNS) to be reclaimed immediately on expiry 2024-01-01 12:38:58 +00:00
sound ALSA: hda/realtek: Add quirk for ASUS ROG GV302XA 2024-01-01 12:39:02 +00:00
tools selftests: mptcp: join: fix subflow_send_ack lookup 2024-01-01 12:39:06 +00:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt kvm/vfio: ensure kvg instance stays around in kvm_vfio_group_add() 2023-09-13 09:42:46 +02:00
.clang-format inet: ping: use hlist_nulls rcu iterator during lookup 2022-12-01 12:42:46 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore Kbuild: add Rust support 2022-09-28 09:02:20 +02:00
.mailmap 9 hotfixes. 6 for MM, 3 for other areas. Four of these patches address 2022-12-10 17:10:52 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Remove Michal Marek from Kbuild maintainers 2022-11-16 14:53:00 +09:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS devlink: move code to a dedicated directory 2023-08-30 16:11:00 +02:00
Makefile Linux 6.1.70 2024-01-01 12:39:09 +00:00
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.