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scatter_data_area() has two purposes:
1) Create the iovs for the data area buffer of a SCSI cmd.
2) If there is data in DMA_TO_DEVICE direction, copy
the data from sg_list to data area buffer.
Both are done in a common loop.
In case of DMA_FROM_DEVICE data transfer, scatter_data_area() is called
with parameter copy_data = false. But this flag is just used to skip
memcpy() for data, while radix_tree_lookup still is called for every dbi of
the area area buffer, and kmap and kunmap are called for every page from
sg_list and data_area as well as flush_dcache_page() for the data area
pages. Since the only thing to do with copy_data = false would be to set
up the iovs, this is a noticeable overhead. Rework the iov creation in the
main loop of scatter_data_area() providing the new function
new_block_to_iov(). Based on this, create the short new function
tcmu_setup_iovs() that only writes the iovs with no overhead. This new
function is now called instead of scatter_data_area() for bidi buffers and
for data buffers in those cases where memcpy() would have been skipped.
Link: https://lore.kernel.org/r/20200910155041.17654-4-bstroesser@ts.fujitsu.com
Acked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.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.