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Colin Foster f0484d2f80 mfd: ocelot-spi: Fix unsupported bulk read 
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
 		2023-04-26 11:40:33 +01:00
Documentation dt-bindings: mfd: qcom,spmi-pmic: Add compatible for pmm8654au 2023-04-26 11:40:33 +01:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
arch - Flush out logged errors immediately after MCA banks configuration 2023-03-19 09:57:53 -07:00
block block: remove obsolete config BLOCK_COMPAT 2023-03-16 09:35:44 -06:00
certs Kbuild updates for v6.3 2023-02-26 11:53:25 -08:00
crypto Networking changes for 6.3. 2023-02-21 18:24:12 -08:00
drivers mfd: ocelot-spi: Fix unsupported bulk read 2023-04-26 11:40:33 +01:00
fs Fix a double unlock bug on an error path in ext4, found by smatch and 2023-03-19 09:38:26 -07:00
include mfd: core: Remove .enable() and .disable() callbacks 2023-04-26 11:40:29 +01:00
init Kbuild updates for v6.3 2023-02-26 11:53:25 -08:00
io_uring io_uring/rsrc: fix folio accounting 2023-03-16 09:32:18 -06:00
ipc Merge branch 'work.namespace' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2023-02-24 19:20:07 -08:00
kernel Tracing fixes for 6.3: 2023-03-19 10:46:02 -07:00
lib Zstd fixes for v6.3 2023-03-14 17:03:25 -07:00
mm mm/damon/paddr: fix folio_nr_pages() after folio_put() in damon_pa_mark_accessed_or_deactivate() 2023-03-07 17:04:55 -08:00
net Including fixes from netfilter, wifi and ipsec. 2023-03-17 13:31:16 -07:00
rust Rust fixes for 6.3-rc1 2023-03-03 14:51:15 -08:00
samples LoongArch changes for v6.3 2023-03-01 09:27:00 -08:00
scripts kbuild: use git-archive for source package creation 2023-03-16 22:46:12 +09:00
security capability: just use a 'u64' instead of a 'u32[2]' array 2023-03-01 10:01:22 -08:00
sound ALSA: hda/ca0132: fixup buffer overrun at tuning_ctl_set() 2023-03-14 17:04:53 +01:00
tools Including fixes from netfilter, wifi and ipsec. 2023-03-17 13:31:16 -07:00
usr usr/gen_init_cpio.c: remove unnecessary -1 values from int file 2022-10-03 14:21:44 -07:00
virt KVM/riscv changes for 6.3 2023-02-15 12:33:28 -05:00
.clang-format cpumask: re-introduce constant-sized cpumask optimizations 2023-03-05 14:30:34 -08:00
.cocciconfig
…
.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 *.dtso files 2023-02-26 15:28:23 +09:00
.gitignore kbuild: rpm-pkg: move source components to rpmbuild/SOURCES 2023-03-16 22:45:56 +09:00
.mailmap Including fixes from netfilter, wifi and ipsec. 2023-03-17 13:31:16 -07:00
.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 There is no particular theme here - mainly quick hits all over the tree. 2023-02-23 17:55:40 -08:00
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 fbdev updates for kernel 6.3-rc3: 2023-03-18 16:01:34 -07:00
Makefile Linux 6.3-rc3 2023-03-19 13:27:55 -07:00
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.