Hi Tom,
The following changes since commit 93330d4ce416208fe202e304e5a18166c57ac569:
Prepare v2020.04-rc4 (2020-03-30 19:29:27 -0400)
are available in the Git repository at:
git@gitlab.denx.de:u-boot/custodians/u-boot-arc.git tags/arc-last-minute-fixes-for-2020.04
for you to fetch changes up to 45bd649511155b5b5a40851ca96036636cb95e23:
ARC: IO: add MB for __raw_* memory accessors (2020-03-31 18:31:53 +0300)
---------------------------------------------------------------- This last minute pull-request is intended to fix some drivers when used on ARC boards. The problem was introduced by https://gitlab.denx.de/u-boot/u-boot/-/commit/07906b3dad157bd58411664bcc6a2a...
What happened while doing one pretty simple improvement to make U-Boot port more flexible and portable (by switching accessors from assembly-written to plain C version) we implicitly added 2 problems:
1. Downgraded accessors from being volatile which signalled to the compiler that it's now possible to do all kinds of optimizations which may easily include merge of subsequent byte reads/writes into word operations. Which is OK for accessing mormal memory but breaks operation of peripherals if we access its memory-mapped regs in such a "creative" manner. 2. As a part of assembly-written implementation we had compiler barriers in form of the following construction 'asm volatile("" : : : "memory")', and we dropped it in C implemntation. This in its turn enabled compiler to mess with instruction ordering. Guess what it gives us in the end :)
So with all that we had in some corner-cases veeery funny instruction flows generated. And in particular it broke DW SPI functionality when we were writing large amount of data. Funny enough our tests which were writing small amount of data still worked and only by the chance we caught that breakage and unrolled that quite interesting loop of unexpected problems.
The road to hell is paved with good intentions. Amen :)
---------------------------------------------------------------- Eugeniy Paltsev (3): ARC: IO: add volatile to accessors ARC: IO: add compiler barriers to IO accessors ARC: IO: add MB for __raw_* memory accessors
arch/arc/include/asm/io.h | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------------------------------- 1 file changed, 59 insertions(+), 33 deletions(-)
Regards, Alexey