linux-kernel - Re: [PATCH] PCI: Fix legacy IRQ assignment execution stage

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Message-ID: <20170929160226.GA17398@red-moon>
Date:   Fri, 29 Sep 2017 17:02:26 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Guenter Roeck <linux@...ck-us.net>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Matt Turner <mattst88@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-arm-kernel@...ts.infradead.org,
        Richard Henderson <rth@...ddle.net>
Subject: Re: [PATCH] PCI: Fix legacy IRQ assignment execution stage

On Thu, Sep 28, 2017 at 05:37:19PM -0500, Bjorn Helgaas wrote:
> On Thu, Sep 28, 2017 at 12:37:07PM +0100, Lorenzo Pieralisi wrote:
> > Through struct pci_host_bridge->{map/swizzle}_irq() hooks is now
> > possible to define IRQ mapping functions on a per PCI host bridge basis.
> > 
> > Actual IRQ allocation is carried out by the pci_assign_irq() function in
> > pci_device_probe() - to make sure a device is assigned an IRQ only if it
> > is probed (ie match a driver); it retrieves a struct pci_host_bridge*
> > for a given PCI device and through {map/swizzle}_irq() hooks it carries
> > out the PCI IRQ allocation.
> > 
> > As it turned out, some legacy drivers (eg IDE layer) require that a
> > device allocates IRQ as soon as it is added so that its actual IRQ
> > settings are available early in the boot process. With current code
> > calling pci_assign_irq() in pci_device_probe() IDE IRQ probing fails
> > for some drivers:
> 
> I think the patch is fine, but I don't understand the changelog.  I
> want to know specifically what the dependency on dev->irq is.  "Early
> in the boot process" is pretty vague.
> 
> I *thought* we were doing something like this:
> 
>   pci_device_probe(dev1)
>     pci_assign_irq(dev1)
>       ...
>         ide_pci_init_two(dev1, dev2, ...)
>           do_ide_setup_pci_device(dev1)
>             pciirq = dev1->irq                # this one is fine
>           do_ide_setup_pci_device(dev2)
>             pciirq = dev2->irq                # not fine
> 
> where the problem is that we haven't called pci_assign_irq(dev2), so
> dev2->irq hasn't been set.
> 
> But that doesn't match the data because we should be coming through
> cmd64x_init_one(), which calls ide_pci_init_one(), so we shouldn't
> have a dev2 in this path.

I *think* I understand what's going on here, the key is:

ide_scan_pcibus()

and CONFIG_IDEPCI_PCIBUS_ORDER

I still have to replicate it but I suspect that
do_ide_setup_pci_device() for dev1 finds an unallocated IRQ (ie dev->irq
== 0) because the probing did NOT happen via pci_device_probe(), ie
pci_device_probe() was not called for the dev1, the cmd64x probe
routine is called straight from ide_scan_pcidev().

I am struggling to understand the logic behind:

ide_pci_register_driver() and ide_scan_pcibus()

and the sequence wrt PCI bus probing but I think that's the problem
and that's why moving pci_assign_irq() to pci_device_add() will
sort this out, adding pci_assign_irq() in ide_scan_pcidev() will
solve the problem too (patch below).

Needless to say, ide_scan_pcibus() relies on pre_init global variable
to make sure ide_pci_register_driver() chooses the "right" way of
registering a driver, see:

__ide_pci_register_driver()

Patch here to verify my assumption in case Guenter has a chance to
run it if I do not beat him to it:

-- >8 --
diff --git a/drivers/ide/ide-scan-pci.c b/drivers/ide/ide-scan-pci.c
index 86aa88a..86b570a 100644
--- a/drivers/ide/ide-scan-pci.c
+++ b/drivers/ide/ide-scan-pci.c
@@ -56,6 +56,8 @@ static int __init ide_scan_pcidev(struct pci_dev *dev)
 {
 	struct list_head *l;
 	struct pci_driver *d;
+	int ret;
+
 
 	list_for_each(l, &ide_pci_drivers) {
 		d = list_entry(l, struct pci_driver, node);
@@ -63,10 +65,14 @@ static int __init ide_scan_pcidev(struct pci_dev *dev)
 			const struct pci_device_id *id =
 				pci_match_id(d->id_table, dev);
 
-			if (id != NULL && d->probe(dev, id) >= 0) {
-				dev->driver = d;
-				pci_dev_get(dev);
-				return 1;
+			if (id != NULL) {
+				pci_assign_irq(dev);
+				ret = d->probe(dev, id);
+				if (ret >= 0) {
+					dev->driver = d;
+					pci_dev_get(dev);
+					return 1;
+				}
 			}
 		}
 	}