PCIe on Cyclone 10 GX: Data loss on DMA writes by FPGA

This post was written by eli on August 15, 2018
Posted Under: Altera,FPGA,PCI express

TL;DR

DMA writes from a Cyclone 10 GX PCIe interface may be lost, probably due to a path that isn’t timed properly by the fitter. This has been observed with Quartus Prime Version 17.1.0 Build 240 SJ Pro Edition, and the official Cyclone 10 GX development board. A wider impact is likely, possibly on Arria 10 device as well (as its PCIe block is the same one).

The problem seems to be rare, and appears and disappears depending on how the fitter places the logic. It’s however fairly easy to diagnose if this specific problem is in effect (see “The smoking gun” below).

Computer hardware: Gigabyte GA-B150M-D2V motherboard (with an Intel B150 Chipset) + Intel i5-6400 CPU.

The story

It started with a routine data transport test (FPGA to host), which failed virtually immediately (that is, after a few kilobytes). It was apparent that some portions of data simply weren’t written into the DMA buffer by the FPGA.

So I tried a fix in my own code, and yep, it helped. Or so I thought. Actually, anything I changed seemed to fix the problem. In the end, I changed nothing, but just added

set_global_assignment -name SEED 2

to the QSF file. Which only changes the fitter’s initial placement of the logic elements, which eventually leads to an alternative placement and routing of the design. That should work exactly the same, of course. But it “solved the problem”.

This was consistent: One “magic” build that failed consistently, and any change whatsoever made the issue disappear.

The design was properly constrained, of course, as shown in the development board’s sample SDC file. In fact, there isn’t much to constrain: It’s just setting the main clock to 100 MHz, derive_pll_clocks and derive_clock_uncertainty. And a false path from the PERST pin.

So maybe my bad? Well, no. There were no unconstrained paths in the entire design (with these simple constraints), so one fitting of the design should be exactly like any other. Maybe my application logic? No again:

The smoking gun

The final nail in the coffin was when I noted errors in the PCIe Device Status Registers on both sides. I’ve discussed this topic in this and this other posts of mine, however in the current case no AER kernel messages were produced (unfortunately, and it’s not clear why).

And whatever the application code does, Intel / Altera’s PCIe block shouldn’t produce a link error, and neither it does normally. It’s a violation of the PCIe spec.

These are the steps for observing this issue on a Linux machine. First, find out who the link partners are:

$ lspci
00:00.0 Host bridge: Intel Corporation Device 191f (rev 07)
00:01.0 PCI bridge: Intel Corporation Device 1901 (rev 07)
[ ... ]
01:00.0 Unassigned class [ff00]: Altera Corporation Device ebeb

and then figuring out that the FPGA card is connected via the bridge at 00:01.0 with

$ lspci -t
-[0000:00]-+-00.0
           +-01.0-[01]----00.0

So it’s between 00:01.0 and 01:00.0. Then, following that post of mine, using setpci to read from the status register to tell an error had occurred.

First, what it should look like: With any bitstream except that specific faulty one, I got

# setpci -s 01:00.0 CAP_EXP+0xa.w
0000
# setpci -s 00:01.0 CAP_EXP+0xa.w
0000

any time and all the time, which says the obvious: No errors sensed on either side.

But with the bitstream that had data losses, before any communication had taken place (except for the driver being loaded):

# setpci -s 01:00.0 CAP_EXP+0xa.w
0009
# setpci -s 00:01.0 CAP_EXP+0xa.w
0000

Non-zero means error. So at this stage the FPGA’s PCIe interface was unhappy with something (more on that below), but the processor’s side had no complaints.

I have to admit that I’ve seen the 0009 status in a lot of other tests, in which communication went through perfectly. So even though reflects some kind of error, it doesn’t necessarily predict any functional fault. As elaborated below, the 0009 status consists of correctable errors. It’s just that such errors are normally never seen (i.e. with any PCIe card that works properly).

Anyhow, back to the bitstream that did have data errors. After some data had been written by the FPGA:

# setpci -s 01:00.0 CAP_EXP+0xa.w
0009
root@diskless:/home/eli# setpci -s 00:01.0 CAP_EXP+0xa.w
000a

In this case, the FPGA card’s link partner complained. To save ourselves the meaning of these numbers (even though the’re listed in that post), use lspci -vv:

# lspci -vv
00:01.0 PCI bridge: Intel Corporation Device 1901 (rev 07) (prog-if 00 [Normal decode])
[ ... ]
        Capabilities: [a0] Express (v2) Root Port (Slot+), MSI 00
                DevCap: MaxPayload 256 bytes, PhantFunc 0
                        ExtTag- RBE+
                DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
                        RlxdOrd- ExtTag- PhantFunc- AuxPwr- NoSnoop-
                        MaxPayload 256 bytes, MaxReadReq 128 bytes
                DevSta: CorrErr- UncorrErr+ FatalErr- UnsuppReq+ AuxPwr- TransPend-
[ ... ]

So the bridge complained about an uncorrectable and an unsupported request only after the data transmission, but the FPGA side:

01:00.0 Unassigned class [ff00]: Altera Corporation Device ebeb
[ ... ]
        Capabilities: [80] Express (v2) Endpoint, MSI 00
                DevCap: MaxPayload 256 bytes, PhantFunc 0, Latency L0s <64ns, L1 <1us
                        ExtTag- AttnBtn- AttnInd- PwrInd- RBE+ FLReset-
                DevCtl: Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
                        RlxdOrd+ ExtTag- PhantFunc- AuxPwr- NoSnoop+
                        MaxPayload 256 bytes, MaxReadReq 512 bytes
                DevSta: CorrErr+ UncorrErr- FatalErr- UnsuppReq+ AuxPwr- TransPend-

complained about a correctable error and an unsupported request (as seen above, that happened before any payload transmission).

Low-level errors. I couldn’t make this happen even if I wanted to.

Aftermath

The really bad news is that this problem isn’t in the logic itself, but in how it’s placed. It seems to be a rare and random occurrence of a poor job done by the fitter. Or maybe it’s not all that rare, if you let the FPGA heat up a bit. In my case a spinning fan kept an almost idle FPGA quite cool, I suppose.

The somewhat good news is that the data loss comes with these PCIe status errors, and maybe with the relevant kernel messages (not clear why I didn’t see any). So there’s something to hold on to.

And I should also mention that the offending PCIe interface was a Gen2 x 4 running with a 64-bit interface at 250 MHz. which a rather marginal frequency for Arria 10 / Cyclone 10. So going with the speculation that this is a timing issue that isn’t handled properly by the fitter, maybe sticking to 125 MHz interfaces on these devices is good enough to be safe against this issue.

Note to self: The outputs are kept in cyclone10-failure.tar.gz

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