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The hardware has a 128 byte RX FIFO buffer for each independent UART. Previously, the code was always reading that byte-by-byte via independent SPI transactions and the associated overhead. In practice, this led to up to eight bytes over SPI for just one byte in the UART's RX FIFO: - reading the global IRQ register (two bytes, one for command, the other for data) - reading one UART's ISR (again two bytes) - reading the byte count (two bytes yet again) - finally, reading one byte of the FIFO via another two-byte transaction We cannot always use a batched read. If the TTY is set to intercept break conditions or report framing or parity errors, then it is required to check the Line Status Register (LSR) for each byte which is read from the RX FIFO. The documentation does not show a way of doing that in a single SPI transaction; registers 0x00 and 0x04 are separate. In my testing, this is no silver bullet. I was feeding 2MB of random data over four daisy-chaned UARTs of MAX14830, and this is the distribution that I was getting: - R <= 1: 7437322 - R <= 2: 162093 - R <= 4: 4093 - R <= 8: 4196 - R <= 16: 645 - R <= 32: 165 - R <= 64: 58 - R <= 128: 0 For a reference, batching the write operations works much better: - W <= 1: 2664 - W <= 2: 1305 - W <= 4: 627 - W <= 8: 371 - W <= 16: 121 - W <= 32: 68 - W <= 64: 33 - W <= 128: 63139 That's probably because this HW/SW combination (Clearfog Base, Armada 388) is probably "good enough" to react to the chip's IRQ "fast enough" most of the time. Still, I was getting RX overruns every now and then. In future, I plan to improve this by letting the RX FIFO be filled a little more (the chip has support for that and also for a "stale timeout" to prevent additional starvation). Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.