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Marc Kleine-Budde 85d4eb2a3d can: bittiming: can_calc_bittiming(): prefer small bit rate pre-scalers over larger ones 
The CiA (CAN in Automation) lists in their Newsletter 1/2018 in the
"Recommendation for the CAN FD bit-timing" [1] article several
recommendations, one of them is:

| Recommendation 3: Choose BRPA and BRPD as low as possible

[1] https://can-newsletter.org/uploads/media/raw/f6a36d1461371a2f86ef0011a513712c.pdf

With the current bit timing algorithm Srinivas Neeli noticed that on
the Xilinx Versal ACAP board the CAN data bit timing parameters are
not calculated optimally. For most bit rates, the bit rate
prescaler (BRP) is != 1, although it's possible to configure the
requested with a bit rate with a prescaler of 1:

| Data Bit timing parameters for xilinx_can_fd2i with 79.999999 MHz ref clock (cmd-line) using algo 'v4.8'
|  nominal                                  real  Bitrt    nom   real  SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP  Error
| 12000000     12   2    2    2   1   1 11428571   4.8%  75.0%  71.4%   4.8%
| 10000000     25   1    1    1   1   2  9999999   0.0%  75.0%  75.0%   0.0%
|  8000000     12   3    3    3   1   1  7999999   0.0%  75.0%  70.0%   6.7%
|  5000000     50   1    1    1   1   4  4999999   0.0%  75.0%  75.0%   0.0%
|  4000000     62   1    1    1   1   5  3999999   0.0%  75.0%  75.0%   0.0%
|  2000000    125   1    1    1   1  10  1999999   0.0%  75.0%  75.0%   0.0%
|  1000000    250   1    1    1   1  20   999999   0.0%  75.0%  75.0%   0.0%

The bit timing parameter calculation algorithm iterates effectively
from low to high BRP values. It selects a new best parameter set, if
the sample point error of the current parameter set is equal or less
to old best parameter set.

If the given hardware constraints (clock rate and bit timing parameter
constants) don't allow a sample point error of 0, the algorithm will
first find a valid bit timing parameter set with a low BRP, but then
will accept parameter sets with higher BRPs that have the same sample
point error.

This patch changes the algorithm to only accept a new parameter set,
if the resulting sample point error is lower. This leads to the
following data bit timing parameter for the Versal ACAP board:

| Data Bit timing parameters for xilinx_can_fd2i with 79.999999 MHz ref clock (cmd-line) using algo 'can-next'
|  nominal                                  real  Bitrt    nom   real  SampP
|  Bitrate TQ[ns] PrS PhS1 PhS2 SJW BRP  Bitrate  Error  SampP  SampP  Error
| 12000000     12   2    2    2   1   1 11428571   4.8%  75.0%  71.4%   4.8%
| 10000000     12   2    3    2   1   1  9999999   0.0%  75.0%  75.0%   0.0%
|  8000000     12   3    3    3   1   1  7999999   0.0%  75.0%  70.0%   6.7%
|  5000000     12   5    6    4   1   1  4999999   0.0%  75.0%  75.0%   0.0%
|  4000000     12   7    7    5   1   1  3999999   0.0%  75.0%  75.0%   0.0%
|  2000000     12  14   15   10   1   1  1999999   0.0%  75.0%  75.0%   0.0%
|  1000000     25  14   15   10   1   2   999999   0.0%  75.0%  75.0%   0.0%

Note: Due to HW constraints a data bit rate of 1 MBit/s with BRP = 1 is not possible.

Link: https://lore.kernel.org/all/20220318144913.873614-1-mkl@pengutronix.de
Link: https://lore.kernel.org/all/20220113203004.jf2rqj2pirhgx72i@pengutronix.de
Cc: Srinivas Neeli <sneeli@xilinx.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
2022-04-19 17:12:05 +02:00
arch Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-04-15 09:26:00 +02:00
block for-5.18/block-2022-04-01 2022-04-01 16:20:00 -07:00
certs Kbuild updates for v5.18 2022-03-31 11:59:03 -07:00
crypto for-5.18/64bit-pi-2022-03-25 2022-03-26 12:01:35 -07:00
Documentation docs: net: dsa: describe issues with checksum offload 2022-04-18 13:29:02 +01:00
drivers can: bittiming: can_calc_bittiming(): prefer small bit rate pre-scalers over larger ones 2022-04-19 17:12:05 +02:00
fs for-5.18-rc2-tag 2022-04-14 10:58:27 -07:00
include can: rx-offload: rename can_rx_offload_queue_sorted() -> can_rx_offload_queue_timestamp() 2022-04-19 16:58:04 +02:00
init Kbuild updates for v5.18 2022-03-31 11:59:03 -07:00
ipc fs: allocate inode by using alloc_inode_sb() 2022-03-22 15:57:03 -07:00
kernel Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-04-15 09:26:00 +02:00
lib Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-04-15 09:26:00 +02:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm - Allow the compiler to optimize away unused percpu accesses and change 2022-04-10 06:56:46 -10:00
net rtnetlink: return EINVAL when request cannot succeed 2022-04-19 13:38:55 +02:00
samples samples, bpf: Move routes monitor in xdp_router_ipv4 in a dedicated thread 2022-04-08 22:10:57 +02:00
scripts hardening fixes for v5.18-rc3 2022-04-12 14:29:40 -10:00
security hardening updates for v5.18-rc1-fix1 2022-03-31 11:43:01 -07:00
sound sound fixes for 5.18-rc3 2022-04-14 11:08:12 -07:00
tools selftests: mlxsw: Introduce devlink line card provision/unprovision/activation tests 2022-04-18 11:00:19 +01:00
usr Kbuild updates for v5.18 2022-03-31 11:59:03 -07:00
virt KVM/arm64 fixes for 5.18, take #1 2022-04-08 12:30:04 -04:00
.clang-format genirq/msi: Make interrupt allocation less convoluted 2021-12-16 22:22:20 +01:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap mailmap: update Vasily Averin's email address 2022-04-08 14:20:36 -10:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: replace a Microchip AT91 maintainer 2022-02-09 11:30:01 +01:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-04-15 09:26:00 +02:00
Makefile Linux 5.18-rc2 2022-04-10 14:21:36 -10:00
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.