On Fri, Dec 13, 2024 at 9:56 PM Niklas Schnelle <niks@xxxxxxxxxx> wrote: > Trying to enable bwctrl on an Intel JHL7540 (Titan Ridge) based > Thunderbolt port causes a boot hang on at least some systems though the > exact reason is not yet understood. As per the spec Thunderbolt PCIe > Downstream Ports have a fake Max Link Speed of 2.5 GT/s (USB4 v2 sec > 11.2.1): > > "Max Link Speed field in the Link Capabilities Register set to 0001b > (data rate of 2.5 GT/s only). > Note: These settings do not represent actual throughput. > Throughput is implementation specific and based on the USB4 Fabric > performance." > > More generally if 2.5 GT/s is the only supported link speed there is no > point in throtteling as this is already the lowest possible PCIe speed > so don't advertise the capability stopping bwctrl from being probed on > these ports. > > The PCIe r6.2 specification section 7.5.3.18 recommends to primarily > utilize the Supported Link Speeds Vector instead of the Max Link Speeds > field to prevent confusion if future specifications allow devices not > to support lower speeds. This concern does not apply however when > specifically targeting devices claiming support only for 2.5 GT/s. > > Link: https://lore.kernel.org/linux-pci/Z1R4VNwCOlh9Sg9n@xxxxxxxxx/ > Fixes: 665745f27487 ("PCI/bwctrl: Re-add BW notification portdrv as PCIe BW controller") > Tested-by: Niklas Schnelle <niks@xxxxxxxxxx> > Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx> > Reviewed-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx> > Signed-off-by: Niklas Schnelle <niks@xxxxxxxxxx> > --- > Note: This issue causes a boot hang on my personal workstation. > > While there is an ongoing discussion about generalizing this to all > devices with a single supported speed. It turns out however that in my > case dev->supported_speeds incorrectly claims 2.5-8 GT/s requiring > a seperate second fix. So in the interest of simplicity and because I'll > be out from the 19th until January, I'd like to propose to do this simple > fix to the boot hang now and take the time to figure out a more general > approach afterwards. > --- > Changes in v2: > - Improve commit message to mention the specific controller and > why using the Max Link Speeds field should be fine here. > - Add a comment (Lukas) > - Add R-b's (no change to logic). > - Link to v1: https://lore.kernel.org/r/20241207-fix_bwctrl_thunderbolt-v1-1-b711f572a705@xxxxxxxxxx This is now commit e50e27a613db6f18 ("PCI/portdrv: Disable bwctrl service if port is fixed at 2.5 GT/s") in pci/next, which conflicts with commit 774c71c52aa48700 ("PCI/bwctrl: Enable only if more than one speed is supported") in v6.13-rc4. Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds
