Hello all.
I'm running a custom kernel based on 5.10.72 on an NXP i.MX8QM (an
ARMv8-A system). The i.MX8QM has 2 designware PCIe 3.0 controllers
on-chip. The driver in use is `drivers/pci/controller/dwc/pci-imx6.c`.
By default the PCIe controller's Link Capabilities are set to support
ASPM L0s only, although the datasheet indicates support for both L0s
and L1. I recently modified the host controller driver to set LnkCap
to indicate ASPM L1 and L0s; one controller is attached to an SSD that
indicates support for L1s, so changing the host controller LnkCap
allowed the `aspm_enabled` bitmask to become non-zero and the ASPM
bits to be set in the LnkCtrl registers on the host and the SSD. This
is the good news.
The bad news is that after the LnkCap change, when I attempt to
suspend-to-RAM, I get an asynchronous exception (an SError to be
specific) at the CPU, resulting in a kernel panic. Here's the tail end
of the pstore log that was captured from the exception and resulting
panic:
```
<6>[ 90.850564] psci: CPU3 killed (polled 4 ms)
<4>[ 90.851989] IRQ 75: no longer affine to CPU4
<4>[ 90.852062] IRQ384: set affinity failed(-22).
<4>[ 90.852065] IRQ386: set affinity failed(-22).
<5>[ 90.852112] CPU4: shutdown
<6>[ 90.853135] psci: CPU4 killed (polled 0 ms)
<4>[ 90.856402] IRQ384: set affinity failed(-22).
<4>[ 90.856409] Eff. affinity 3-5 of IRQ 386 contains only offline
CPUs after offlining CPU 5
<2>[ 90.856536] SError Interrupt on CPU5, code 0xbf000002 -- SError
<4>[ 90.856538] CPU: 5 PID: 37 Comm: migration/5 Tainted: G
C O 5.10.72-lts-5.10.y+g886272a218dd #1
<4>[ 90.856539] Hardware name: Freescale i.MX8QM, Rivian TCM Board X1 (DT)
<4>[ 90.856541] pstate: 00000085 (nzcv daIf -PAN -UAO -TCO BTYPE=--)
<4>[ 90.856542] pc : msi_set_mask_bit.isra.0+0x40/0x90
<4>[ 90.856544] lr : msi_set_mask_bit.isra.0+0x34/0x90
<4>[ 90.856545] sp : ffff800012743bf0
<4>[ 90.856546] x29: ffff800012743bf0 x28: 0000000000000005
<4>[ 90.856550] x27: ffff000806260218 x26: ffff8000121c9260
<4>[ 90.856553] x25: ffff800011d1b000 x24: ffff8000121c9480
<4>[ 90.856556] x23: ffff80001225a768 x22: ffff000806260260
<4>[ 90.856559] x21: ffff0008062602dc x20: ffff000806260200
<4>[ 90.856562] x19: ffff00080286b200 x18: 0000000000000020
<4>[ 90.856565] x17: 0000000000000001 x16: 0000000000000019
<4>[ 90.856567] x15: ffff0008023812f8 x14: 6e696e696c66666f
<4>[ 90.856570] x13: 2072657466612073 x12: 55504320656e696c
<4>[ 90.856572] x11: 66666f20796c6e6f x10: 20736e6961746e6f
<4>[ 90.856575] x9 : ffff8000106aa084 x8 : 4920666f20352d33
<4>[ 90.856578] x7 : 207974696e696666 x6 : 0000000000000000
<4>[ 90.856581] x5 : 0000000000000000 x4 : 0000000000000000
<4>[ 90.856584] x3 : 0000000000000001 x2 : ffff800012d6302c
<4>[ 90.856586] x1 : 0000000000000001 x0 : 0000000000000000
<0>[ 90.856589] Kernel panic - not syncing: Asynchronous SError Interrupt
<4>[ 90.856591] CPU: 5 PID: 37 Comm: migration/5 Tainted: G
C O 5.10.72-lts-5.10.y+g886272a218dd #1
<4>[ 90.856592] Hardware name: Freescale i.MX8QM
<4>[ 90.856594] Call trace:
<4>[ 90.856595] dump_backtrace+0x0/0x1b0
<4>[ 90.856596] show_stack+0x24/0x30
<4>[ 90.856597] dump_stack+0xd0/0x12c
<4>[ 90.856598] panic+0x178/0x380
<4>[ 90.856600] nmi_panic+0x98/0xa0
<4>[ 90.856601] arm64_serror_panic+0x8c/0x98
<4>[ 90.856602] do_serror+0x64/0x6c
<4>[ 90.856603] el1_error+0x90/0x110
<4>[ 90.856604] msi_set_mask_bit.isra.0+0x40/0x90
<4>[ 90.856605] pci_msi_mask_irq+0x2c/0x40
<4>[ 90.856606] dw_msi_mask_irq+0x24/0x40
<4>[ 90.856607] irq_shutdown+0xa4/0xe0
<4>[ 90.856609] irq_shutdown_and_deactivate+0x24/0x3c
<4>[ 90.856610] irq_migrate_all_off_this_cpu+0x260/0x290
<4>[ 90.856611] __cpu_disable+0xd8/0xf0
<4>[ 90.856612] take_cpu_down+0x48/0xf0
<4>[ 90.856613] multi_cpu_stop+0xb4/0x1a0
<4>[ 90.856614] cpu_stopper_thread+0xa0/0x130
<4>[ 90.856615] smpboot_thread_fn+0x25c/0x290
<4>[ 90.856616] kthread+0x164/0x16c
<4>[ 90.856617] ret_from_fork+0x10/0x30
<2>[ 91.856635] SMP: stopping secondary CPUs
<4>[ 91.856636] SMP: failed to stop secondary CPUs 0
<0>[ 91.856637] Kernel Offset: disabled
<0>[ 91.856639] CPU features: 0x0240022,2100600c
<0>[ 91.856640] Memory Limit: none
```
Note that the exception occurs right around when we're masking MSIs,
which happens because CPU5 goes offline and the kernel recognizes that
there are no more CPUs left to handle those interrupts. To be a little
more specific, the PC indicates that the CPU was executing a data
memory barrier when the exception arrived. Source here [1] and
disassembly below with my annotation showing the PC when the exception
hit:
```
1b64: b9005260 str w0, [x19, #80]
asm volatile(ALTERNATIVE("ldr %w0, [%1]",
1b68: f9403260 ldr x0, [x19, #96]
1b6c: b9400000 ldr w0, [x0]
readl(desc->mask_base); /* Flush write to
device */
1b70: d5033dbf dmb ld <--- PC during exception
1b74: 2a0003e0 mov w0, w0
1b78: ca000000 eor x0, x0, x0
```
At the point of the exception I believe that the PCIe controller is
powered down, although I haven't confirmed.
- - -
I'm trying to understand what's going wrong but I've hit a wall. I
thought the act of writing to the MSI_MASK bit while the HC is powered
down is what caused the issue, but I hacked a fix to avoid calling
`msi_set_mask_bit()` during suspend-to-RAM and still got the
exception.
At this point I'm wondering why we mask MSIs so late in the suspend
process (right when the last non-boot CPU is taken offline). Shouldn't
we disable/mask these IRQs as part of host controller suspend?
I'm also wondering if maybe the PCIe _device_ - the SSD - is writing
into host controller memory after the HC is powered down, which could
cause an exception on the data bus that would obviously be
asynchronous to the CPU. But I've just started learning about PCIe and
I have a very fuzzy understanding of how data flows during an MSI.
Any advice would be welcome.
Thanks,
Kevin
[1] https://source.codeaurora.org/external/imx/linux-imx/tree/drivers/pci/msi.c?h=lf-5.10.y#n243
