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For RISC-V platforms we permit assigning addresses from 0 to PCI devices, both in the memory and the I/O bus space, and we happily do so if there is no conflict, e.g.: pci 0000:07:00.0: BAR 0: assigned [io 0x0000-0x0007] pci 0000:07:00.1: BAR 0: assigned [io 0x0008-0x000f] pci 0000:06:01.0: PCI bridge to [bus 07] pci 0000:06:01.0: bridge window [io 0x0000-0x0fff] (with the SiFive HiFive Unmatched RISC-V board and a dual serial port option card based on the OxSemi OXPCIe952 device wired for the legacy UART mode). Address 0 is treated specially however in many places, for example in `pci_iomap_range' and `pci_iomap_wc_range' we require that the start address is non-zero, and even if we let such an address through, then individual device drivers could reject a request to handle a device at such an address, such as in `uart_configure_port'. Consequently given devices configured as shown above only one is actually usable: Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled serial 0000:07:00.0: enabling device (0000 -> 0001) serial: probe of 0000:07:00.0 failed with error -12 serial 0000:07:00.1: enabling device (0000 -> 0001) serial 0000:07:00.1: detected caps 00000700 should be 00000500 0000:07:00.1: ttyS0 at I/O 0x8 (irq = 39, base_baud = 15625000) is a 16C950/954 Therefore avoid handing out address 0, by bumping the lowest address available to PCI via PCIBIOS_MIN_IO and PCIBIOS_MIN_MEM up by 4 and 16 respectively, which is the minimum allocation size for I/O and memory BARs. With this in place the system in question we have: pci 0000:07:00.0: BAR 0: assigned [io 0x1000-0x1007] pci 0000:07:00.1: BAR 0: assigned [io 0x1008-0x100f] pci 0000:06:01.0: PCI bridge to [bus 07] pci 0000:06:01.0: bridge window [io 0x1000-0x1fff] and then devices work correctly: Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled serial 0000:07:00.0: enabling device (0000 -> 0001) serial 0000:07:00.0: detected caps 00000700 should be 00000500 0000:07:00.0: ttyS0 at I/O 0x1000 (irq = 38, base_baud = 15625000) is a 16C950/954 serial 0000:07:00.1: enabling device (0000 -> 0001) serial 0000:07:00.1: detected caps 00000700 should be 00000500 0000:07:00.1: ttyS1 at I/O 0x1008 (irq = 39, base_baud = 15625000) is a 16C950/954 Especially I/O space ranges are particularly valuable, because bridges only decode bits from 12 up and consequently where 16-bit addressing is in effect, as few as 16 separate ranges can be assigned to individual buses only, however a generic change to avoid handing out address 0 only has turned out controversial as per the discussion referred via the link below. Conversely sorting this out in platform code has been standard practice since forever to avoid a clash with legacy devices subtractively decoded by the southbridge where present. This can be revised should a generic solution be adopted sometime. Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk> Link: https://lore.kernel.org/r/alpine.DEB.2.21.2202260044180.25061@angie.orcam.me.uk Signed-off-by: Palmer Dabbelt <palmer@rivosinc.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.