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The source for the Linux kernel used in Windows Subsystem for Linux 2 (WSL2)
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Maciej W. Rozycki 6f47a7594b serial: 8250: Add proper clock handling for OxSemi PCIe devices 
[ Upstream commit 366f6c955d ]

Oxford Semiconductor PCIe (Tornado) 950 serial port devices are driven
by a fixed 62.5MHz clock input derived from the 100MHz PCI Express clock.

We currently drive the device using its default oversampling rate of 16
and the clock prescaler disabled, consequently yielding the baud base of
3906250.  This base is inadequate for some of the high-speed baud rates
such as 460800bps, for which the closest rate possible can be obtained
by dividing the baud base by 8, yielding the baud rate of 488281.25bps,
which is off by 5.9638%.  This is enough for data communication to break
with the remote end talking actual 460800bps, where missed stop bits
have been observed.

We can do better however, by taking advantage of a reduced oversampling
rate, which can be set to any integer value from 4 to 16 inclusive by
programming the TCR register, and by using the clock prescaler, which
can be set to any value from 1 to 63.875 in increments of 0.125 in the
CPR/CPR2 register pair.  The prescaler has to be explicitly enabled
though by setting bit 7 in the MCR or otherwise it is bypassed (in the
enhanced mode that we enable) as if the value of 1 was used.

Make use of these features then as follows:

- Set the baud base to 15625000, reflecting the minimum oversampling
  rate of 4 with the clock prescaler and divisor both set to 1.

- Override the `set_mctrl' and set the MCR shadow there so as to have
  MCR[7] always set and have the 8250 core propagate these settings.

- Override the `get_divisor' handler and determine a good combination of
  parameters by using a lookup table with predetermined value pairs of
  the oversampling rate and the clock prescaler and finding a pair that
  divides the input clock such that the quotient, when rounded to the
  nearest integer, deviates the least from the exact result.  Calculate
  the clock divisor accordingly.

  Scale the resulting oversampling rate (only by powers of two) if
  possible so as to maximise it, reducing the divisor accordingly, and
  avoid a divisor overflow for very low baud rates by scaling the
  oversampling rate and/or the prescaler even if that causes some
  accuracy loss.

  Also handle the historic spd_cust feature so as to allow one to set
  all the three parameters manually to arbitrary values, by keeping the
  low 16 bits for the divisor and then putting TCR in bits 19:16 and
  CPR/CPR2 in bits 28:20, sanitising the bit pattern supplied such as
  to clamp CPR/CPR2 values between 0.000 and 0.875 inclusive to 33.875.
  This preserves compatibility with any existing setups, that is where
  requesting a custom divisor that only has any bits set among the low
  16 the oversampling rate of 16 and the clock prescaler of 33.875 will
  be used as with the original 8250.

  Finally abuse the `frac' argument to store the determined bit patterns
  for the TCR, CPR and CPR2 registers.

- Override the `set_divisor' handler so as to set the TCR, CPR and CPR2
  registers from the `frac' value supplied.  Set the divisor as usual.

With the baud base set to 15625000 and the unsigned 16-bit UART_DIV_MAX
limitation imposed by `serial8250_get_baud_rate' standard baud rates
below 300bps become unavailable in the regular way, e.g. the rate of
200bps requires the baud base to be divided by 78125 and that is beyond
the unsigned 16-bit range.  The historic spd_cust feature can still be
used to obtain such rates if so required.

See Documentation/tty/device_drivers/oxsemi-tornado.rst for more details.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181519450.9383@angie.orcam.me.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
 		2022-08-17 14:24:23 +02:00
Documentation serial: 8250: Add proper clock handling for OxSemi PCIe devices 2022-08-17 14:24:23 +02:00
LICENSES LICENSES/dual/CC-BY-4.0: Git rid of "smart quotes" 2021-07-15 06:31:24 -06:00
arch KVM: x86: Signal #GP, not -EPERM, on bad WRMSR(MCi_CTL/STATUS) 2022-08-17 14:24:22 +02:00
block block: ensure iov_iter advances for added pages 2022-08-17 14:24:01 +02:00
certs certs/blacklist_hashes.c: fix const confusion in certs blacklist 2022-06-22 14:22:01 +02:00
crypto crypto: blake2s - remove shash module 2022-08-17 14:24:19 +02:00
drivers serial: 8250: Add proper clock handling for OxSemi PCIe devices 2022-08-17 14:24:23 +02:00
fs __follow_mount_rcu(): verify that mount_lock remains unchanged 2022-08-17 14:24:19 +02:00
include crypto: blake2s - remove shash module 2022-08-17 14:24:19 +02:00
init stack: Declare {randomize_,}kstack_offset to fix Sparse warnings 2022-08-17 14:23:10 +02:00
ipc ipc/mqueue: use get_tree_nodev() in mqueue_get_tree() 2022-06-09 10:23:10 +02:00
kernel sched/core: Do not requeue task on CPU excluded from cpus_mask 2022-08-17 14:24:15 +02:00
lib crypto: blake2s - remove shash module 2022-08-17 14:24:19 +02:00
mm mm/mmap.c: fix missing call to vm_unacct_memory in mmap_region 2022-08-17 14:23:58 +02:00
net net: 9p: fix refcount leak in p9_read_work() error handling 2022-08-17 14:24:08 +02:00
samples samples/landlock: Format with clang-format 2022-06-09 10:23:23 +02:00
scripts scripts/faddr2line: Fix vmlinux detection on arm64 2022-08-17 14:24:14 +02:00
security selinux: Add boundary check in put_entry() 2022-08-17 14:23:05 +02:00
sound ASoC: mchp-spdifrx: disable end of block interrupt on failures 2022-08-17 14:24:11 +02:00
tools tools/thermal: Fix possible path truncations 2022-08-17 14:24:15 +02:00
usr usr/include/Makefile: add linux/nfc.h to the compile-test coverage 2022-02-01 17:27:15 +01:00
virt KVM: Don't set Accessed/Dirty bits for ZERO_PAGE 2022-08-17 14:23:44 +02:00
.clang-format clang-format: Update with the latest for_each macro list 2021-05-12 23:32:39 +02:00
.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: add Andrej Shadura 2021-10-18 20:22:03 -10:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
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 Input: goodix - add a goodix.h header file 2022-07-12 16:34:51 +02:00
Makefile Makefile: link with -z noexecstack --no-warn-rwx-segments 2022-08-17 14:22:44 +02:00
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.