On Thu, Jul 18, 2019 at 10:38 AM Timur Kristóf <timur.kristof@xxxxxxxxx> wrote: > > > > > > > > I took a look at amdgpu_device_get_pcie_info() and found that it > > > uses > > > pcie_bandwidth_available to determine the capabilities of the PCIe > > > port. However, pcie_bandwidth_available gives you only the current > > > bandwidth as set by the PCIe link status register, not the maximum > > > capability. > > > > > > I think something along these lines would fix it: > > > https://pastebin.com/LscEMKMc > > > > > > It seems to me that the PCIe capabilities are only used in a few > > > places > > > in the code, so this patch fixes pp_dpm_pcie. However it doesn't > > > affect > > > the actual performance. > > > > > > What do you think? > > > > I think we want the current bandwidth. The GPU can only control the > > speed of its local link. If there are upstream links that are slower > > than its local link, it doesn't make sense to run the local link at > > faster speeds because it will burn extra power it will just run into > > a > > bottleneck at the next link. In general, most systems negotiate the > > fastest link speed supported by both ends at power up. > > > > Alex > > Currently, if the GPU connected to a TB3 port, the driver thinks that > 2.5 GT/s is the best speed that it can use, even though the hardware > itself uses 8 GT/s. So what the driver thinks is inconsistent with what > the hardware does. This messes up pp_dpm_pcie. > > As far as I understand, PCIe bridge devices can change their link speed > in runtime based on how they are used or what power state they are in, > so it makes sense here to request the best speed they are capable of. I > might be wrong about that. I don't know of any bridges off hand that change their link speeds on demand. That said, I'm certainly not a PCI expert. Our GPUs for instance have a micro-controller on them which changes the speed on demand. Presumably other devices would need something similar. > > If you think this change is undesireable, then maybe it would be worth > to follow Mika's suggestion and add something along the lines of > dev->is_thunderbolt so that the correct available bandwidth could still > be determined. Ideally, it would be added to the core pci helpers so that each driver that uses them doesn't have to duplicate the same functionality. Alex > > Tim > _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
