Re: [PATCH 1/2] drm/amdgpu: return the PCIe gen and lanes from the INFO

[Marek Olšák <maraeo@xxxxxxxxx>]

On Wed, Jan 4, 2023 at 6:50 AM Lazar, Lijo <lijo.lazar@xxxxxxx> wrote:

On 1/4/2023 4:11 AM, Marek Olšák wrote:
> I see. Well, those sysfs files are not usable, and I don't think it
> would be important even if they were usable, but for completeness:
>
> The ioctl returns:
>      pcie_gen = 1
>      pcie_num_lanes = 16
>
> Theoretical bandwidth from those values: 4.0 GB/s
> My DMA test shows this write bandwidth: 3.5 GB/s
> It matches the expectation.
>
> Let's see the devices (there is only 1 GPU Navi21 in the system):
> $ lspci |egrep '(PCI|VGA).*Navi'
> 0a:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL
> Upstream Port of PCI Express Switch (rev c3)
> 0b:00.0 PCI bridge: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 XL
> Downstream Port of PCI Express Switch
> 0c:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
> [AMD/ATI] Navi 21 [Radeon RX 6800/6800 XT / 6900 XT] (rev c3)
>
> Let's read sysfs:
>
> $ cat /sys/bus/pci/devices/0000:0a:00.0/current_link_width
> 16
> $ cat /sys/bus/pci/devices/0000:0b:00.0/current_link_width
> 16
> $ cat /sys/bus/pci/devices/0000:0c:00.0/current_link_width
> 16
> $ cat /sys/bus/pci/devices/0000:0a:00.0/current_link_speed
> 2.5 GT/s PCIe
> $ cat /sys/bus/pci/devices/0000:0b:00.0/current_link_speed
> 16.0 GT/s PCIe
> $ cat /sys/bus/pci/devices/0000:0c:00.0/current_link_speed
> 16.0 GT/s PCIe
>
> Problem 1: None of the speed numbers match 4 GB/s.

US bridge = 2.5GT/s means operating at PCIe Gen 1 speed. Total
theoretical bandwidth is then derived based on encoding and total number
of lanes.

> Problem 2: Userspace doesn't know the bus index of the bridges, and it's
> not clear which bridge should be used.

In general, modern ones have this arch= US->DS->EP. US is the one
connected to physical link.

> Problem 3: The PCIe gen number is missing.

Current link speed is based on whether it's Gen1/2/3/4/5.

BTW, your patch makes use of capabilities flags which gives the maximum
supported speed/width by the device. It may not necessarily reflect the
current speed/width negotiated. I guess in NV, this info is already
obtained from PMFW and made available through metrics table.

It computes the minimum of the device PCIe gen and the motherboard/slot PCIe gen to get the final value. These 2 lines do that. The low 16 bits of the mask contain the device PCIe gen mask. The high 16 bits of the mask contain the slot PCIe gen mask.

+ pcie_gen_mask = adev->pm.pcie_gen_mask & (adev->pm.pcie_gen_mask >> 16);

+ dev_info->pcie_gen = fls(pcie_gen_mask);

Marek