Re: [RESEND PATCH v4] x86/PCI: Recognize that Interrupt Line 255 means "not connected"

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jan 29, 2016 at 11:40:14AM +0800, Chen Fan wrote:
> Per the x86-specific footnote to PCI spec r3.0, sec 6.2.4, the value 255 in
> the Interrupt Line register means "unknown" or "no connection."
> Previously, when we couldn't derive an IRQ from the _PRT, we fell back to
> using the value from Interrupt Line as an IRQ.  It's questionable whether
> we should do that at all, but the spec clearly suggests we shouldn't do it
> for the value 255 on x86.
> 
> Calling request_irq() with IRQ 255 may succeed, but the driver won't
> receive any interrupts.  Or, if IRQ 255 is shared with another device, it
> may succeed, and the driver's ISR will be called at random times when the
> *other* device interrupts.  Or it may fail if another device is using IRQ
> 255 with incompatible flags.  What we *want* is for request_irq() to fail
> predictably so the driver can fall back to polling.
> 
> On x86, assume 255 in the Interrupt Line means the INTx line is not
> connected.  In that case, set dev->irq to IRQ_NOTCONNECTED so request_irq()
> will fail gracefully with -ENOTCONN.
> 
> We found this problem on a system where Secure Boot firmware assigned
> Interrupt Line 255 to an i801_smbus device and another device was already
> using MSI-X IRQ 255.  This was in v3.10, where i801_probe() fails if
> request_irq() fails:
> 
>   i801_smbus 0000:00:1f.3: enabling device (0140 -> 0143)
>   i801_smbus 0000:00:1f.3: can't derive routing for PCI INT C
>   i801_smbus 0000:00:1f.3: PCI INT C: no GSI
>   genirq: Flags mismatch irq 255. 00000080 (i801_smbus) vs. 00000000 (megasa)
>   CPU: 0 PID: 2487 Comm: kworker/0:1 Not tainted 3.10.0-229.el7.x86_64 #1
>   Hardware name: FUJITSU PRIMEQUEST 2800E2/D3736, BIOS PRIMEQUEST 2000 Serie5
>   Call Trace:
>     dump_stack+0x19/0x1b
>     __setup_irq+0x54a/0x570
>     request_threaded_irq+0xcc/0x170
>     i801_probe+0x32f/0x508 [i2c_i801]
>     local_pci_probe+0x45/0xa0
>   i801_smbus 0000:00:1f.3: Failed to allocate irq 255: -16
>   i801_smbus: probe of 0000:00:1f.3 failed with error -16
> 
> After aeb8a3d16ae0 ("i2c: i801: Check if interrupts are disabled"),
> i801_probe() will fall back to polling if request_irq() fails.  But we
> still need this patch because request_irq() may succeed or fail depending
> on other devices in the system.  If request_irq() fails, i801_smbus will
> work by falling back to polling, but if it succeeds, i801_smbus won't work
> because it expects interrupts that it may not receive.
> 
> Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
> Acked-by: Thomas Gleixner <tglx@linutronix.de>
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>

Hi Chen,

You already have my ack, but I'm just curious if you can tell whether
i801_smbus works if IRQ 255 isn't in use, e.g., if you can remove the
megasa driver or tell it not to use MSI?

Bjorn