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In general each DMA-based SPI transfer can be split up into two stages: DMA data transmission/reception and SPI-bus transmission/reception. DMA asynchronous transactions completion can be tracked by means of the DMA async Tx-descriptor completion callback. But that callback being called indicates that the DMA transfer has been finished, it doesn't mean that SPI data transmission is also done. Moreover in fact it isn't for at least Tx-only SPI transfers. Upon DMA transfer completion some data is left in the Tx FIFO and being pushed out by the SPI controller. So in order to make sure that an SPI transfer is completely pushed to the SPI-bus, the driver has to wait for both DMA transaction and the SPI-bus transmission/reception are finished. Note if there is a way to asynchronously track the former event by means of the DMA async Tx callback, there isn't easy one for the later (IRQ-based solution won't work since SPI controller doesn't notify about Rx FIFO being empty). The DMA transfer completion callback isn't suitable to wait for the SPI controller activity finish either. The callback might (in case of DW DMAC it will) be called in the tasklet context. Waiting for the SPI controller to complete the transfer might take a considerable amount of time since SPI-bus might be pretty slow. In this case delaying the execution in the tasklet atomic context might cause significant system performance drop. So to speak the best option we've got to solve the problem is to consequently wait for both stages being finished in the locally implemented SPI transfer execution procedure even if it costs us of the local wait-function re-implementation. In this case we don't need to use the SPI-core transfer-wait functionality, but we'll make sure that all DMA and SPI-bus transactions are completely finished before the SPI-core transfer_one callback returns. In this commit we provide an implementation of the DMA-transfers completion wait functionality. The DW APB SSI DMA-specific SPI transfer_one function waits for both Tx and Rx DMA transfers being finished, and only then exits with zero returned signalling to the SPI core that the SPI transfer is finished. This implementation is fully equivalent to the currently used DMA-execution-SPI-core-wait algorithm. The SPI-bus transmission/reception wait methods will be added in the follow-up commits. Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru> Cc: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru> Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru> Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru> Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Feng Tang <feng.tang@intel.com> Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com> Cc: Rob Herring <robh+dt@kernel.org> Cc: linux-mips@vger.kernel.org Cc: devicetree@vger.kernel.org Link: https://lore.kernel.org/r/20200529131205.31838-4-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.