pci_alloc_irq_vectors fails ENOSPC for XPS 13 9310

[Bjorn Helgaas]

[+cc Govind, author of 5697a564d369 ("ath11k: pci: add MSI config
initialisation")]

On Mon, Nov 02, 2020 at 08:49:51PM +0200, Kalle Valo wrote:
> + linux-wireless, linux-pci, devin
> 
> Thomas Krause <thomaskrause at posteo.de> writes:
> 
> >> I had the same problem as well back in the days, for me enabling
> >> CONFIG_IRQ_REMAP helped. If it helps for you also I wonder if we should
> >> mention that in the ath11k warning above :)
> >
> > CONFIG_IRQ_REMAP did not do the trick. I noticed that the Wi-Fi card
> > is behind a PCI bridge which is also disabled, could this be a
> > problem?
> >
> > 00:1c.0 PCI bridge: Intel Corporation Device a0b8 (rev 20) (prog-if 00
> > [Normal decode])
> > 	Flags: bus master, fast devsel, latency 0, IRQ 123
> > 	Bus: primary=00, secondary=56, subordinate=56, sec-latency=0
> > 	I/O behind bridge: [disabled]
> > 	Memory behind bridge: 8c300000-8c3fffff [size=1M]
> > 	Prefetchable memory behind bridge: [disabled]
> > 	Capabilities: [40] Express Root Port (Slot+), MSI 00
> > 	Capabilities: [80] MSI: Enable+ Count=1/1 Maskable- 64bit-
> > 	Capabilities: [90] Subsystem: Dell Device 0991
> > 	Capabilities: [a0] Power Management version 3
> > 	Capabilities: [100] Advanced Error Reporting
> > 	Capabilities: [220] Access Control Services
> > 	Capabilities: [150] Precision Time Measurement
> > 	Capabilities: [200] L1 PM Substates
> > 	Capabilities: [a00] Downstream Port Containment
> > 	Kernel driver in use: pcieport
> 
> I don't know enough about PCI to say if the bridge is a problem or not.

I don't think the bridge is an issue here.  AFAICT the bridge's I/O
and prefetchable memory windows are disabled, but the non-prefetchable
window *is* enabled and contains the space consumed by the ath11k
device:

  00:1c.0 PCI bridge: Intel Corporation Device a0b8 (rev 20)
	Bus: primary=00, secondary=56, subordinate=56, sec-latency=0
	Memory behind bridge: 8c300000-8c3fffff [size=1M]
  56:00.0 Network controller: Qualcomm Device 1101 (rev 01)
     Region 0: Memory at 8c300000 (64-bit, non-prefetchable) [size=1M]

> To summarise: Thomas is reporting[1] a problem with ath11k on QCA6390
> PCI device where he is not having enough MSI vectors. ath11k needs 32
> vectors but pci_alloc_irq_vectors() returns -ENOSPC. PCI support is new
> for ath11k and introduced in v5.10-rc1. The irq allocation code is in
> drivers/net/wireless/ath/ath11k/pci.c. [2]

This code is needlessly complicated.  If you absolutely need
msi_config.total_vectors and can't settle for any less, you can do
this:

  num_vectors = pci_alloc_irq_vectors(ab_pci->pdev,
                                      msi_config.total_vectors,
                                      msi_config.total_vectors,
                                      PCI_IRQ_MSI);

  if (num_vectors < 0) {
    ath11k_err(ab, "failed to get %d MSI vectors (%d)\n",
               msi_config.total_vectors, num_vectors);
    return num_vectors;
  }

But it seems a little greedy if the device can't operate at all unless
it gets 32 vectors.  Are you sure that's a hard requirement?  Most
devices can work with fewer vectors, even if it reduces performance.

> I would first try with a full distro kernel config, just in case there's
> some another important kernel config missing.
> 
> [1] http://lists.infradead.org/pipermail/ath11k/2020-October/000466.html

Tangent: have you considered getting this list archived on
https://lore.kernel.org/lists.html?

> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/wireless/ath/ath11k/pci.c#n633
> 
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