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Colin Foster d617022971 mfd: ocelot-spi: Fix unsupported bulk read 
[ Upstream commit f0484d2f80 ]

Ocelot chips (VSC7511, VSC7512, VSC7513, VSC7514) don't support bulk read
operations over SPI.

Many SPI buses have hardware that can optimize consecutive reads.
Essentially an address is written to the chip, and if the SPI controller
continues to toggle the clock, subsequent register values are reported.
This can lead to significant optimizations, because the time between
"address is written to the chip" and "chip starts to report data" can often
take a fixed amount of time.

When support for Ocelot chips were added in commit f3e893626a ("mfd:
ocelot: Add support for the vsc7512 chip via spi") it was believed that
this optimization was supported. However it is not.

Most register transactions with the Ocelot chips are not done in bulk, so
this bug could go unnoticed. The one scenario where bulk register
operations _are_ performed is when polling port statistics counters, which
was added in commit d87b1c08f3 ("net: mscc: ocelot: use bulk reads for
stats").

Things get slightly more complicated here...

A bug was introduced in commit d4c3676507 ("net: mscc: ocelot: keep
ocelot_stat_layout by reg address, not offset") that broke the optimization
of bulk reads. This means that when Ethernet support for the VSC7512 chip
was added in commit 3d7316ac81 ("net: dsa: ocelot: add external ocelot
switch control") things were actually working "as expected".

The bulk read opmtimization was discovered, and fixed in commit
6acc72a43e ("net: mscc: ocelot: fix stats region batching") and the
timing optimizations for SPI were noticed. A bulk read went from ~14ms to
~2ms. But this timing improvement came at the cost of every register
reading zero due the fact that bulk reads don't work.

The read timings increase back to 13-14ms, but that's a price worth paying
in order to receive valid data. This is verified in a DSA setup (cpsw-new
switch tied to port 0 on the VSC7512, after having been running overnight)

     Rx Octets: 16222055 # Counters from CPSW switch
     Tx Octets: 12034702
     Net Octets: 28256757
     p00_rx_octets: 12034702 # Counters from Ocelot switch
     p00_rx_frames_below_65_octets: 0
     p00_rx_frames_65_to_127_octets: 88188
     p00_rx_frames_128_to_255_octets: 13
     p00_rx_frames_256_to_511_octets: 0
     p00_rx_frames_512_to_1023_octets: 0
     p00_rx_frames_over_1526_octets: 3306
     p00_tx_octets: 16222055

Fixes: f3e893626a ("mfd: ocelot: Add support for the vsc7512 chip via spi")
Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230322141130.2531256-1-colin.foster@in-advantage.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2023-05-11 23:03:38 +09:00
arch openrisc: Properly store r31 to pt_regs on unhandled exceptions 2023-05-11 23:03:35 +09:00
block blk-mq: don't plug for head insertions in blk_execute_rq_nowait 2023-05-11 23:03:22 +09:00
certs certs: Fix build error when PKCS#11 URI contains semicolon 2023-02-09 11:28:11 +01:00
crypto crypto: drbg - Only fail when jent is unavailable in FIPS mode 2023-05-11 23:03:21 +09:00
Documentation ASoC: dt-bindings: qcom,lpass-rx-macro: correct minItems for clocks 2023-05-11 23:03:05 +09:00
drivers mfd: ocelot-spi: Fix unsupported bulk read 2023-05-11 23:03:38 +09:00
fs ext4: fix use-after-free read in ext4_find_extent for bigalloc + inline 2023-05-11 23:03:36 +09:00
include SUNRPC: remove the maximum number of retries in call_bind_status 2023-05-11 23:03:35 +09:00
init gcc: disable '-Warray-bounds' for gcc-13 too 2023-04-26 14:28:43 +02:00
io_uring io_uring/rsrc: use nospec'ed indexes 2023-05-11 23:03:24 +09:00
ipc ipc: fix memory leak in init_mqueue_fs() 2022-12-31 13:32:01 +01:00
kernel timekeeping: Fix references to nonexistent ktime_get_fast_ns() 2023-05-11 23:03:35 +09:00
lib debugobject: Prevent init race with static objects 2023-05-11 23:03:16 +09:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm mm/mempolicy: fix use-after-free of VMA iterator 2023-05-01 08:26:27 +09:00
net SUNRPC: remove the maximum number of retries in call_bind_status 2023-05-11 23:03:35 +09:00
rust rust: kernel: Mark rust_fmt_argument as extern "C" 2023-04-26 14:28:38 +02:00
samples ftrace: Export ftrace_free_filter() to modules 2023-02-01 08:34:37 +01:00
scripts scripts/gdb: raise error with reduced debugging information 2023-05-11 23:03:30 +09:00
security selinux: ensure av_permissions.h is built when needed 2023-05-11 23:03:06 +09:00
sound ASoC: fsl_mqs: move of_node_put() to the correct location 2023-05-11 23:03:29 +09:00
tools tracing/user_events: Ensure write index cannot be negative 2023-05-11 23:03:32 +09:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM: Register /dev/kvm as the _very_ last thing during initialization 2023-03-10 09:34:11 +01:00
.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 audit: update the mailing list in MAINTAINERS 2023-02-25 11:25:42 +01:00
Makefile Linux 6.1.27 2023-05-01 08:26:28 +09:00
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.