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NVMe devices can advertise multiple power states. These states can be either "operational" (the device is fully functional but possibly slow) or "non-operational" (the device is asleep until woken up). Some devices can automatically enter a non-operational state when idle for a specified amount of time and then automatically wake back up when needed. The hardware configuration is a table. For each state, an entry in the table indicates the next deeper non-operational state, if any, to autonomously transition to and the idle time required before transitioning. This patch teaches the driver to program APST so that each successive non-operational state will be entered after an idle time equal to 100% of the total latency (entry plus exit) associated with that state. The maximum acceptable latency is controlled using dev_pm_qos (e.g. power/pm_qos_latency_tolerance_us in sysfs); non-operational states with total latency greater than this value will not be used. As a special case, setting the latency tolerance to 0 will disable APST entirely. On hardware without APST support, the sysfs file will not be exposed. The latency tolerance for newly-probed devices is set by the module parameter nvme_core.default_ps_max_latency_us. In theory, the device can expose "default" APST table, but this doesn't seem to function correctly on my device (Samsung 950), nor does it seem particularly useful. There is also an optional mechanism by which a configuration can be "saved" so it will be automatically loaded on reset. This can be configured from userspace, but it doesn't seem useful to support in the driver. On my laptop, enabling APST seems to save nearly 1W. The hardware tables can be decoded in userspace with nvme-cli. 'nvme id-ctrl /dev/nvmeN' will show the power state table and 'nvme get-feature -f 0x0c -H /dev/nvme0' will show the current APST configuration. This feature is quirked off on a known-buggy Samsung device. Signed-off-by: Andy Lutomirski <luto@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Sagi Grimberg <sagi@grimberg.me> Signed-off-by: Jens Axboe <axboe@fb.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.