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A relatively standard idiom for ensuring that a pair of MMIO writes to a device arrive at that device with a specified minimum delay between them is as follows: writel_relaxed(42, dev_base + CTL1); readl(dev_base + CTL1); udelay(10); writel_relaxed(42, dev_base + CTL2); the intention being that the read-back from the device will push the prior write to CTL1, and the udelay will hold up the write to CTL1 until at least 10us have elapsed. Unfortunately, on arm64 where the underlying delay loop is implemented as a read of the architected counter, the CPU does not guarantee ordering from the readl() to the delay loop and therefore the delay loop could in theory be speculated and not provide the desired interval between the two writes. Fix this in a similar manner to PowerPC by introducing a dummy control dependency on the output of readX() which, combined with the ISB in the read of the architected counter, guarantees that a subsequent delay loop can not be executed until the readX() has returned its result. Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Arnd Bergmann <arnd@arndb.de> Signed-off-by: Will Deacon <will.deacon@arm.com>
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.