mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-14 06:24:53 +08:00
08839c06e9
This reworks the SPI transfer implementation to make use of more of the SPI core features. The main benefit is to avoid the memcpy in sja1105_xfer_buf(). The memcpy was only needed because the function was transferring a single buffer at a time. So it needed to copy the caller-provided buffer at buf + 4, to store the SPI message header in the "headroom" area. But the SPI core supports scatter-gather messages, comprised of multiple transfers. We can actually use those to break apart every SPI message into 2 transfers: one for the header and one for the actual payload. To keep the behavior the same regarding the chip select signal, it is necessary to tell the SPI core to de-assert the chip select after each chunk. This was not needed before, because each spi_message contained only 1 single transfer. The meaning of the per-transfer cs_change=1 is: - If the transfer is the last one of the message, keep CS asserted - Otherwise, deassert CS We need to deassert CS in the "otherwise" case, which was implicit before. Avoiding the memcpy creates yet another opportunity. The device can't process more than 256 bytes of SPI payload at a time, so the sja1105_xfer_long_buf() function used to exist, to split the larger caller buffer into chunks. But these chunks couldn't be used as scatter/gather buffers for spi_message until now, because of that memcpy (we would have needed more memory for each chunk). So we can now remove the sja1105_xfer_long_buf() function and have a single implementation for long and short buffers. Another benefit is lower usage of stack memory. Previously we had to store 2 SPI buffers for each chunk. Due to the elimination of the memcpy, we can now send pointers to the actual chunks from the caller-supplied buffer to the SPI core. Since the patch merges two functions into a rewritten implementation, the function prototype was also changed, mainly for cosmetic consistency with the structures used within it. Signed-off-by: Vladimir Oltean <olteanv@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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.