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In case if at least one of the requested DMA engine channels doesn't support the hardware accelerated SG list entries traverse, the DMA driver will most likely work that around by performing the IRQ-based SG list entries resubmission. That might and will cause a problem if the DMA Tx channel is recharged and re-executed before the Rx DMA channel. Due to non-deterministic IRQ-handler execution latency the DMA Tx channel will start pushing data to the SPI bus before the Rx DMA channel is even reinitialized with the next inbound SG list entry. By doing so the DMA Tx channel will implicitly start filling the DW APB SSI Rx FIFO up, which while the DMA Rx channel being recharged and re-executed will eventually be overflown. In order to solve the problem we have to feed the DMA engine with SG list entries one-by-one. It shall keep the DW APB SSI Tx and Rx FIFOs synchronized and prevent the Rx FIFO overflow. Since in general the SPI tx_sg and rx_sg lists may have different number of entries of different lengths (though total length should match) we virtually split the SG-lists to the set of DMA transfers, which length is a minimum of the ordered SG-entries lengths. The solution described above is only executed if a full-duplex SPI transfer is requested and the DMA engine hasn't provided channels with hardware accelerated SG list traverse capability to handle both SG lists at once. Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Link: https://lore.kernel.org/r/20200920112322.24585-12-Sergey.Semin@baikalelectronics.ru Signed-off-by: Mark Brown <broonie@kernel.org> |
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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.