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Attempt to handle cases such as with a downstream port of the ASMedia ASM2824 PCIe switch where link training never completes and the link continues switching between speeds indefinitely with the data link layer never reaching the active state. It has been observed with a downstream port of the ASMedia ASM2824 Gen 3 switch wired to the upstream port of the Pericom PI7C9X2G304 Gen 2 switch, using a Delock Riser Card PCI Express x1 > 2 x PCIe x1 device, P/N 41433, wired to a SiFive HiFive Unmatched board. In this setup the switches should negotiate a link speed of 5.0GT/s, falling back to 2.5GT/s if necessary. Instead the link continues oscillating between the two speeds, at the rate of 34-35 times per second, with link training reported repeatedly active ~84% of the time. Limiting the target link speed to 2.5GT/s with the upstream ASM2824 device makes the two switches communicate correctly. Removing the speed restriction afterwards makes the two devices switch to 5.0GT/s then. Make use of these observations and detect the inability to train the link by checking for the Data Link Layer Link Active status bit being off while the Link Bandwidth Management Status indicating that hardware has changed the link speed or width in an attempt to correct unreliable link operation. Restrict the speed to 2.5GT/s then with the Target Link Speed field, request a retrain and wait 200ms for the data link to go up. If this is successful, lift the restriction, letting the devices negotiate a higher speed. Also check for a 2.5GT/s speed restriction the firmware may have already arranged and lift it too with ports of devices known to continue working afterwards (currently only ASM2824), that already report their data link being up. [bhelgaas: reorder and squash stubs from https://lore.kernel.org/r/alpine.DEB.2.21.2306111619570.64925@angie.orcam.me.uk to avoid adding stubs that do nothing] Link: https://lore.kernel.org/r/alpine.DEB.2.21.2203022037020.56670@angie.orcam.me.uk/ Link: https://source.denx.de/u-boot/u-boot/-/commit/a398a51ccc68 Link: https://lore.kernel.org/r/alpine.DEB.2.21.2305310038540.59226@angie.orcam.me.uk Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com> |
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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.