On Tue, 2025-01-14 at 09:50 +0100, Geert Uytterhoeven wrote: > 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 Oh, that's not right, good catch. This patch is superseded by "PCI/bwctrl: Enable only if more than one speed is supported" and should be dropped. Sorry if that wasn't clear from the the limited context of this patch discussion. @Bjorn I think this needs to be taken care of by you? Thanks, Niklas
