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Brian Norris bb276262e8 mtd: spi-nor: only apply reset hacks to broken hardware 
Commit 59b356ffd0 ("mtd: m25p80: restore the status of SPI flash when
exiting") is the latest from a long history of attempts to add reboot
handling to handle stateful addressing modes on SPI flash. Some prior
mostly-related discussions:

http://lists.infradead.org/pipermail/linux-mtd/2013-March/046343.html
[PATCH 1/3] mtd: m25p80: utilize dedicated 4-byte addressing commands

http://lists.infradead.org/pipermail/barebox/2014-September/020682.html
[RFC] MTD m25p80 3-byte addressing and boot problem

http://lists.infradead.org/pipermail/linux-mtd/2015-February/057683.html
[PATCH 2/2] m25p80: if supported put chip to deep power down if not used

Previously, attempts to add reboot-time software reset handling were
rejected, but the latest attempt was not.

Quick summary of the problem:
Some systems (e.g., boot ROM or bootloader) assume that they can read
initial boot code from their SPI flash using 3-byte addressing. If the
flash is left in 4-byte mode after reset, these systems won't boot. The
above patch provided a shutdown/remove hook to attempt to reset the
addressing mode before we reboot. Notably, this patch misses out on
huge classes of unexpected reboots (e.g., crashes, watchdog resets).

Unfortunately, it is essentially impossible to solve this problem 100%:
if your system doesn't know how to reset the SPI flash to power-on
defaults at initialization time, no amount of software can really rescue
you -- there will always be a chance of some unexpected reset that
leaves your flash in an addressing mode that your boot sequence didn't
expect.

While it is not directly harmful to perform hacks like the
aforementioned commit on all 4-byte addressing flash, a
properly-designed system should not need the hack -- and in fact,
providing this hack may mask the fact that a given system is indeed
broken. So this patch attempts to apply this unsound hack more narrowly,
providing a strong suggestion to developers and system designers that
this is truly a hack. With luck, system designers can catch their errors
early on in their development cycle, rather than applying this hack long
term. But apparently enough systems are out in the wild that we still
have to provide this hack.

Document a new device tree property to denote systems that do not have a
proper hardware (or software) reset mechanism, and apply the hack (with
a loud warning) only in this case.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
2018-08-01 09:27:38 +02:00
arch Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
block for-linus-20180616 2018-06-17 05:37:55 +09:00
certs docs: Fix some broken references 2018-06-15 18:10:01 -03:00
crypto docs: Fix some broken references 2018-06-15 18:10:01 -03:00
Documentation mtd: spi-nor: only apply reset hacks to broken hardware 2018-08-01 09:27:38 +02:00
drivers mtd: spi-nor: only apply reset hacks to broken hardware 2018-08-01 09:27:38 +02:00
firmware kbuild: remove all dummy assignments to obj- 2017-11-18 11:46:06 +09:00
fs Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
include mtd: spi-nor: only apply reset hacks to broken hardware 2018-08-01 09:27:38 +02:00
init Kbuild updates for v4.18 (2nd) 2018-06-13 08:40:34 -07:00
ipc ipc: use new return type vm_fault_t 2018-06-15 07:55:25 +09:00
kernel Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
lib docs: Fix some broken references 2018-06-15 18:10:01 -03:00
LICENSES LICENSES: Add Linux-OpenIB license text 2018-04-27 16:41:53 -06:00
mm mm: fix oom_kill event handling 2018-06-15 07:55:25 +09:00
net Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2018-06-16 07:39:34 +09:00
samples VFIO updates for v4.18 2018-06-12 13:11:26 -07:00
scripts scripts/documentation-file-ref-check: check tools/*/Documentation 2018-06-15 18:10:01 -03:00
security docs: Fix some broken references 2018-06-15 18:10:01 -03:00
sound docs: Fix some broken references 2018-06-15 18:10:01 -03:00
tools Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
usr kbuild: rename built-in.o to built-in.a 2018-03-26 02:01:19 +09:00
virt - Error path bug fix for overflow tests (Dan) 2018-06-12 18:28:00 -07:00
.clang-format clang-format: add configuration file 2018-04-11 10:28:35 -07:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.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 branch 'asoc-4.17' into asoc-4.18 for compress dependencies 2018-04-26 12:24:28 +01:00
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 kconfig: add basic helper macros to scripts/Kconfig.include 2018-05-29 03:31:19 +09:00
MAINTAINERS Solve a series of broken links for files under Documentation: 2018-06-17 05:25:18 +09:00
Makefile Linux 4.18-rc1 2018-06-17 08:04:49 +09:00
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.