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Lukas Wunner 4c524191c0
spi: bcm2835: Work around DONE bit erratum 
Commit 3bd7f6589f ("spi: bcm2835: Overcome sglist entry length
limitation") amended the BCM2835 SPI driver with support for DMA
transfers whose buffers are not aligned to 4 bytes and require more than
one sglist entry.

When testing this feature with upcoming commits to speed up TX-only and
RX-only transfers, I noticed that SPI transmission sometimes breaks.
A function introduced by the commit, bcm2835_spi_transfer_prologue(),
performs one or two PIO transmissions as a prologue to the actual DMA
transmission.  It turns out that the breakage goes away if the DONE bit
in the CS register is set when ending such a PIO transmission.

The DONE bit signifies emptiness of the TX FIFO.  According to the spec,
the bit is of type RO, so writing it should never have any effect.
Perhaps the spec is wrong and the bit is actually of type RW1C.
E.g. the I2C controller on the BCM2835 does have an RW1C DONE bit which
needs to be cleared by the driver.  Another, possibly more likely
explanation is that it's a hardware erratum since the issue does not
occur consistently.

Either way, amend bcm2835_spi_transfer_prologue() to always write the
DONE bit.

Usually a transmission is ended by bcm2835_spi_reset_hw().  If the
transmission was successful, the TX FIFO is empty and thus the DONE bit
is set when bcm2835_spi_reset_hw() reads the CS register.  The bit is
then written back to the register, so we happen to do the right thing.

However if DONE is not set, e.g. because transmission is aborted with
a non-empty TX FIFO, the bit won't be written by bcm2835_spi_reset_hw()
and it seems possible that transmission might subsequently break.  To be
on the safe side, likewise amend bcm2835_spi_reset_hw() to always write
the bit.

Tested-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Martin Sperl <kernel@martin.sperl.org>
Link: https://lore.kernel.org/r/edb004dff4af6106f6bfcb89e1a96391e96eb857.1564825752.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>
 		2019-09-10 11:28:44 +01:00
Documentation spi: Fix the number of CS lines documented as an example 2019-08-20 13:17:59 +01:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
arch s390 updates for 5.2-rc4 2019-06-08 13:12:54 -07:00
block block, bfq: add weight symlink to the bfq.weight cgroup parameter 2019-06-07 01:29:40 -06:00
certs treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 36 2019-05-24 17:27:11 +02:00
crypto SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
drivers spi: bcm2835: Work around DONE bit erratum 2019-09-10 11:28:44 +01:00
fs A change to call iput() asynchronously to avoid a possible deadlock 2019-06-08 15:57:35 -07:00
include spi: Add a prototype for exported spi_set_cs_timing() 2019-06-17 13:49:56 +01:00
init Char/Misc driver fixes for 5.2-rc4 2019-06-08 12:50:36 -07:00
ipc treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
kernel SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
lib SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
mm treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
net SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
samples SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
scripts SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
security treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
sound treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 441 2019-06-05 17:37:17 +02:00
tools SPDX update for 5.2-rc4 2019-06-08 12:52:42 -07:00
usr user/Makefile: Fix typo and capitalization in comment section 2018-12-11 00:18:03 +09:00
virt treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 333 2019-06-05 17:37:06 +02:00
.clang-format Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-04-17 11:26:25 -07:00
.cocciconfig
…
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore .gitignore: exclude .get_maintainer.ignore and .gitattributes 2019-05-18 11:49:54 +09:00
.mailmap A reasonably busy cycle for docs, including: 2019-05-08 12:42:50 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Farewell Martin Schwidefsky 2019-05-31 10:14:11 +02:00
Kbuild Kbuild updates for v5.1 2019-03-10 17:48:21 -07:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS Linux 5.2-rc4 2019-06-10 18:52:53 +01:00
Makefile Linux 5.2-rc4 2019-06-08 20:24:46 -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.