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

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Message-ID: <20240726113008.GE2628@thinkpad>
Date: Fri, 26 Jul 2024 17:00:08 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Siddharth Vadapalli <s-vadapalli@...com>
Cc: 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 Fri, Jul 26, 2024 at 03:54:17PM +0530, Siddharth Vadapalli wrote:
> On Thu, Jul 25, 2024 at 02:01:48PM +0530, Siddharth Vadapalli wrote:
> > 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.
> 
> Mani,
> 
> I prefer setting 'swizzle_irq' to NULL as well unless you have an objection
> to it. Kindly let me know. I plan to post the v2 for this patch addressing
> Bjorn's feedback and collecting Andrew's "Tested-by" tag as well.
> 

Ok, fine with me.

- Mani

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