9f41ba1dff
[ Upstream commit ef302283ddfceaba2657923af3f90fd58e6dff06 ]
mlx5 has a built in self-test at driver startup to evaluate if the
platform supports write combining to generate a 64 byte PCIe TLP or
not. This has proven necessary because a lot of common scenarios end up
with broken write combining (especially inside virtual machines) and there
is other way to learn this information.
This self test has been consistently failing on new ARM64 CPU
designs (specifically with NVIDIA Grace's implementation of Neoverse
V2). The C loop around writeq() generates some pretty terrible ARM64
assembly, but historically this has worked on a lot of existing ARM64 CPUs
till now.
We see it succeed about 1 time in 10,000 on the worst effected
systems. The CPU architects speculate that the load instructions
interspersed with the stores makes the WC buffers statistically flush too
often and thus the generation of large TLPs becomes infrequent. This makes
the boot up test unreliable in that it indicates no write-combining,
however userspace would be fine since it uses a ST4 instruction.
Further, S390 has similar issues where only the special zpci_memcpy_toio()
will actually generate large TLPs, and the open coded loop does not
trigger it at all.
Fix both ARM64 and S390 by switching to __iowrite64_copy() which now
provides architecture specific variants that have a high change of
generating a large TLP with write combining. x86 continues to use a
similar writeq loop in the generate __iowrite64_copy().
Fixes: 11f552e21755 ("IB/mlx5: Test write combining support")
Link: https://lore.kernel.org/r/6-v3-1893cd8b9369+1925-mlx5_arm_wc_jgg@nvidia.com
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Acked-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
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.