I am a bit confused concerning an issue I had with a PCI Express (PCIe) card used to interface serial device on a Linux computer. The card is based on the XR17V354 chip from MaxLinear.
From the beginning, the card was correctly recognized by the OS (it was listed in the
lspci output), but reading and writing to the serial device (
/dev/ttyXRx) failed, no matter which driver was used (8250 serial driver built into the Linux kernel or device/chip manufacturer driver).
Now, I eventually traced back the problem to an interrupt issue: interrupts were correctly raised on the card (visible by polling the UART ISR register and also in the PCI INTx register), but never "reached" the CPU (counter in
/proc/interrupt stuck at 0).
The issue was solved by disabling MSI (Message Signaled Interrupts), by adding
pci=nomsi in the Linux kernel command line parameters.
Now, this is the part that confuses me:
- I read that PCIe devices are required to support MSI
PCI Express permits devices to use these legacy interrupt messages,
retaining software compatibility with PCI drivers, but they are
required to also support MSI or MSI-X in the PCI layer.
- In the chip datasheet and also in the
lspci -voutput, I see that there is an entry in the PCI Configuration Registers for the MSI capability (MSI Capacity ID = 0x05).
- However, all the associated registers (eg Message Control Register) are Read-Only… which explains why MSI is not working and I had to disable it in the OS so it would handle INTx interrupts raised by the device
Does it mean the device is "broken", ie not compliant to PCIe requirements?
Or does the MSI support required from PCIe devices just implies to have a MSI Capacity ID (0x05), but doesn't actually require to implement the MSI feature?