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Dave Martin 4a7e625ce5 tty: pl011: Avoid spuriously stuck-off interrupts 
Commit 9b96fbacda ("serial: PL011: clear pending interrupts")
clears the RX and receive timeout interrupts on pl011 startup, to
avoid a screaming-interrupt scenario that can occur when the
firmware or bootloader leaves these interrupts asserted.

This has been noted as an issue when running Linux on qemu [1].

Unfortunately, the above fix seems to lead to potential
misbehaviour if the RX FIFO interrupt is asserted _non_ spuriously
on driver startup, if the RX FIFO is also already full to the
trigger level.

Clearing the RX FIFO interrupt does not change the FIFO fill level.
In this scenario, because the interrupt is now clear and because
the FIFO is already full to the trigger level, no new assertion of
the RX FIFO interrupt can occur unless the FIFO is drained back
below the trigger level.  This never occurs because the pl011
driver is waiting for an RX FIFO interrupt to tell it that there is
something to read, and does not read the FIFO at all until that
interrupt occurs.

Thus, simply clearing "spurious" interrupts on startup may be
misguided, since there is no way to be sure that the interrupts are
truly spurious, and things can go wrong if they are not.

This patch instead clears the interrupt condition by draining the
RX FIFO during UART startup, after clearing any potentially
spurious interrupt.  This should ensure that an interrupt will
definitely be asserted if the RX FIFO subsequently becomes
sufficiently full.

The drain is done at the point of enabling interrupts only.  This
means that it will occur any time the UART is newly opened through
the tty layer.  It will not apply to polled-mode use of the UART by
kgdboc: since that scenario cannot use interrupts by design, this
should not matter.  kgdboc will interact badly with "normal" use of
the UART in any case: this patch makes no attempt to paper over
such issues.

This patch does not attempt to address the case where the RX FIFO
fills faster than it can be drained: that is a pathological
hardware design problem that is beyond the scope of the driver to
work around.  As a failsafe, the number of poll iterations for
draining the FIFO is limited to twice the FIFO size.  This will
ensure that the kernel at least boots even if it is impossible to
drain the FIFO for some reason.

[1] [Qemu-devel] [Qemu-arm] [PATCH] pl011: do not put into fifo
before enabled the interruption
https://lists.gnu.org/archive/html/qemu-devel/2018-01/msg06446.html

Reported-by: Wei Xu <xuwei5@hisilicon.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Peter Maydell <peter.maydell@linaro.org>
Fixes: 9b96fbacda ("serial: PL011: clear pending interrupts")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Cc: stable <stable@vger.kernel.org>
Tested-by: Wei Xu <xuwei5@hisilicon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2018-05-14 13:41:05 +02:00
arch Merge 4.17-rc3 into tty-next 2018-04-30 05:14:55 -07:00
block Revert "blk-mq: remove code for dealing with remapping queue" 2018-04-25 09:49:22 -06:00
certs certs/blacklist_nohashes.c: fix const confusion in certs blacklist 2018-02-21 15:35:43 -08:00
crypto crypto: drbg - set freed buffers to NULL 2018-04-21 00:57:00 +08:00
Documentation Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-29 09:03:25 -07:00
drivers tty: pl011: Avoid spuriously stuck-off interrupts 2018-05-14 13:41:05 +02:00
firmware kbuild: remove all dummy assignments to obj- 2017-11-18 11:46:06 +09:00
fs Fix misc. bugs and a regression for ext4. 2018-04-28 20:07:21 -07:00
include Merge 4.17-rc3 into tty-next 2018-04-30 05:14:55 -07:00
init Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-15 16:12:35 -07:00
ipc ipc/shm: fix use-after-free of shm file via remap_file_pages() 2018-04-13 17:10:27 -07:00
kernel Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-29 09:03:25 -07:00
lib Driver core fixes for 4.17-rc3 2018-04-27 10:12:20 -07:00
LICENSES LICENSES: Add MPL-1.1 license 2018-01-06 10:59:44 -07:00
mm x86/pti: Filter at vma->vm_page_prot population 2018-04-25 11:02:51 +02:00
net A CephFS quota follow-up and fixes for two older issues in the 2018-04-27 10:56:29 -07:00
samples Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jikos/livepatching 2018-04-20 08:51:55 -07:00
scripts Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
security Merge branch 'userns-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2018-04-24 17:58:51 -07:00
sound ALSA: dice: fix error path to destroy initialized stream data 2018-04-26 15:24:54 +02:00
tools Merge branch 'x86-pti-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-29 09:36:22 -07:00
usr kbuild: rename built-in.o to built-in.a 2018-03-26 02:01:19 +09:00
virt KVM fixes for v4.17-rc3 2018-04-27 16:13:31 -07:00
.clang-format clang-format: add configuration file 2018-04-11 10:28:35 -07:00
.cocciconfig
.get_maintainer.ignore
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore Kbuild updates for v4.17 (2nd) 2018-04-15 17:21:30 -07:00
.mailmap Merge candidates for 4.17 merge window 2018-04-06 17:35:43 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS/CREDITS: Drop METAG ARCHITECTURE 2018-03-05 16:34:24 +00:00
Kbuild Kbuild updates for v4.15 2017-11-17 17:45:29 -08:00
Kconfig License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
MAINTAINERS MAINTAINERS: add myself as maintainer of AFFS 2018-04-28 10:30:47 -07:00
Makefile Linux v4.17-rc3 2018-04-29 14:17:42 -07:00
README Docs: Added a pointer to the formatted docs to README 2018-03-21 09:02:53 -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.
See Documentation/00-INDEX for a list of what is contained in each file.

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.