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For platforms that have xHCI controllers attached over PCIe, and non-coherent routes to main memory, a theoretical race exists between posting new TRBs to a ring, and writing to the doorbell register. In a contended system, write traffic from the CPU may be stalled before the memory controller, whereas the CPU to Endpoint route is separate and not likely to be contended. Similarly, the DMA route from the endpoint to main memory may be separate and uncontended. Therefore the xHCI can receive a doorbell write and find a stale view of a transfer ring. In cases where only a single TRB is ping-ponged at a time, this can cause the endpoint to not get polled at all. Adding a readl() before the write forces a round-trip transaction across PCIe, definitively serialising the CPU along the PCI producer-consumer ordering rules. Signed-off-by: Jonathan Bell <jonathan@raspberrypi.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.
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