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[ Upstream commit 04516706bb99889986ddfa3a769ed50d2dc7ac13 ] When we read device memory, we lock a spinlock, write the address we want to read from the device and then spin in a loop reading the data in 32-bit quantities from another register. As the description makes clear, this is rather inefficient, incurring a PCIe bus transaction for every read. In a typical device today, we want to read 786k SMEM if it crashes, leading to 192k register reads. Occasionally, we've seen the whole loop take over 20 seconds and then triggering the soft lockup detector. Clearly, it is unreasonable to spin here for such extended periods of time. To fix this, break the loop down into an outer and an inner loop, and break out of the inner loop if more than half a second elapsed. To avoid too much overhead, check for that only every 128 reads, though there's no particular reason for that number. Then, unlock and relock to obtain NIC access again, reprogram the start address and continue. This will keep (interrupt) latencies on the CPU down to a reasonable time. Signed-off-by: Johannes Berg <johannes.berg@intel.com> Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com> Signed-off-by: Luca Coelho <luciano.coelho@intel.com> Signed-off-by: Kalle Valo <kvalo@codeaurora.org> Link: https://lore.kernel.org/r/iwlwifi.20201022165103.45878a7e49aa.I3b9b9c5a10002915072312ce75b68ed5b3dc6e14@changeid 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.