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Previously the code did the work implied by the given SCSI command and after that it waited for a timer based on the user specified command duration to be exhausted before informing the mid-level that the command was complete. For short command durations, the time to complete the work implied by the SCSI command could be significant compared to the user specified command duration. For example a WRITE of 128 blocks (say 512 bytes each) on a machine that can copy from main memory to main memory at a rate of 10 GB/sec will take around 6.4 microseconds to do that copy. If the user specified a command duration of 5 microseconds (ndelay=5000), should the driver do a further delay of 5 microseconds after the copy or return immediately because 6.4 > 5 ? The action prior to this patch was to always do the timer based delay. After this patch, for ndelay values less than 1 millisecond, this driver will complete the command immediately. And in the case where the user specified delay was 7 microseconds, a timer delay of 600 nanoseconds will be set ((7 - 6.4) * 1000). Link: https://lore.kernel.org/r/20200421151424.32668-6-dgilbert@interlog.com Signed-off-by: Douglas Gilbert <dgilbert@interlog.com> Signed-off-by: Martin K. Petersen <martin.petersen@oracle.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.