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77218e83c8
ioread8() operations to TPM MMIO addresses can stall the CPU when immediately following a sequence of iowrite*()'s to the same region. For example, cyclitest measures ~400us latency spikes when a non-RT usermode application communicates with an SPI-based TPM chip (Intel Atom E3940 system, PREEMPT_RT kernel). The spikes are caused by a stalling ioread8() operation following a sequence of 30+ iowrite8()s to the same address. I believe this happens because the write sequence is buffered (in CPU or somewhere along the bus), and gets flushed on the first LOAD instruction (ioread*()) that follows. The enclosed change appears to fix this issue: read the TPM chip's access register (status code) after every iowrite*() operation to amortize the cost of flushing data to chip across multiple instructions. Signed-off-by: Haris Okanovic <haris.okanovic@ni.com> Link: https://lore.kernel.org/r/20230323153436.B2SATnZV@linutronix.de Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Tested-by: Jarkko Sakkinen <jarkko@kernel.org> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> Signed-off-by: Jarkko Sakkinen <jarkko@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.