linux-kernel - Re: [PATCH] PCI: j721e: Set .map_irq and .swizzle

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Message-ID: <5f7328f8-eabc-4a8c-87a3-b27e2f6c0c1f@ti.com>
Date: Thu, 25 Jul 2024 14:01:48 +0530
From: Siddharth Vadapalli <s-vadapalli@...com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
CC: Siddharth Vadapalli <s-vadapalli@...com>, <bhelgaas@...gle.com>,
        <lpieralisi@...nel.org>, <kw@...ux.com>, <robh@...nel.org>,
        <vigneshr@...com>, <kishon@...nel.org>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
        <stable@...r.kernel.org>, <ahalaney@...hat.com>, <srk@...com>
Subject: Re: [PATCH] PCI: j721e: Set .map_irq and .swizzle_irq to NULL

On Thu, Jul 25, 2024 at 01:17:08PM +0530, Manivannan Sadhasivam wrote:
> On Thu, Jul 25, 2024 at 10:50:13AM +0530, Siddharth Vadapalli wrote:
> > On Wed, Jul 24, 2024 at 09:49:16PM +0530, Manivannan Sadhasivam wrote:
> > > On Wed, Jul 24, 2024 at 12:20:48PM +0530, Siddharth Vadapalli wrote:
> > > > Since the configuration of Legacy Interrupts (INTx) is not supported, set
> > > > the .map_irq and .swizzle_irq callbacks to NULL. This fixes the error:
> > > >   of_irq_parse_pci: failed with rc=-22
> > > > due to the absence of Legacy Interrupts in the device-tree.
> > > > 
> > > 
> > > Do you really need to set 'swizzle_irq' to NULL? pci_assign_irq() will bail out
> > > if 'map_irq' is set to NULL.
> > 
> > While 'swizzle_irq' won't be invoked if 'map_irq' is NULL, having a
> > non-NULL 'swizzle_irq' (pci_common_swizzle in this case) with a NULL
> > 'map_irq' seems inconsistent to me though the code-path may never invoke
> > it. Wouldn't a non-NULL 'swizzle_irq' imply that Legacy Interrupts are
> > supported, while a NULL 'map_irq' indicates that they aren't? Since they
> > are always described in pairs, whether it is in the initial commit that
> > added support for the Cadence PCIe Host controller (used by pci-j721e.c):
> > https://github.com/torvalds/linux/commit/1b79c5284439
> > OR the commit which moved the shared 'map_irq' and 'swizzle_irq' defaults
> > from all the host drivers into the common 'devm_of_pci_bridge_init()'
> > function:
> > https://github.com/torvalds/linux/commit/b64aa11eb2dd
> > I have set both of them to NULL for the sake of consistency.
> > 
> 
> Since both callbacks are populated in the pci/of driver, this consistency won't
> be visible in the controller drivers. From the functionality pov, setting both
> callbacks to NULL is *not* required to disable INTx, right?

Yes, setting 'swizzle_irq' to NULL isn't required. The execution sequence
with 'swizzle_irq' set to 'pci_common_swizzle()' is as follows:

pci_assign_irq()
  if (pin) {
    if (hbrg->swizzle_irq)
      slot = (*(hbrg->swizzle_irq))(dev, &pin);
        pci_common_swizzle()
	  while (!pci_is_root_bus(dev->bus)) <= NOT entered
	..continue execution similar to 'swizzle_irq' being NULL.

Having 'swizzle_irq' set to 'pci_common_swizzle()' will only result
in a no-op which could have been avoided by setting it to NULL. So there
is no difference w.r.t. functionality.

Regards,
Siddharth.