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Re: [PATCH] OF: PCI: const usage needed by MIPS

To: Bjorn Helgaas <bhelgaas@google.com>
Subject: Re: [PATCH] OF: PCI: const usage needed by MIPS
From: David Daney <ddaney.cavm@gmail.com>
Date: Thu, 03 May 2012 18:17:59 -0700
Cc: John Crispin <blogic@openwrt.org>, Ralf Baechle <ralf@linux-mips.org>, Grant Likely <grant.likely@secretlab.ca>, linux-pci@vger.kernel.org, devicetree-discuss@lists.ozlabs.org, linux-mips@linux-mips.org
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On 05/03/2012 05:30 PM, Bjorn Helgaas wrote:
On Tue, May 1, 2012 at 7:28 AM, John Crispin<blogic@openwrt.org>  wrote:
On 30/04/12 19:54, David Daney wrote:
On 04/30/2012 10:46 AM, John Crispin wrote:
On MIPS we want to call of_irq_map_pci from inside

arch/mips/include/asm/pci.h:extern int pcibios_map_irq(
                 const struct pci_dev *dev, u8 slot, u8 pin);

For this to work we need to change several functions to const usage.

I think there is a mismatch on this throughout the kernel.

Properly fixing it requires touching many more places than these.
Although I haven't tried it, I wouldn't be surprised if doing this
caused warnings to appear in non-MIPS code.

Ralf had a patch at one point that tried to make this consistent
tree-wide, but it is not yet applied.

David Daney

Hi,

Ok, lets see what Ralf has to say.

I just tested the patch on x86 with OF enabled and drivers turned on
that use the API. I did not see any errors appear.

I'm far from a const expert, but I think this should be safe.

Here's my reasoning:

We're changing pci_swizzle_interrupt_pin() to take a pointer to a
constant struct pci_dev.  pci_swizzle_interrupt_pin() only reads the
struct pci_dev; it doesn't modify it.  It is legal to pass either
"struct pci_dev *" or "const struct pci_dev *" to a function expecting
"const struct pci_dev *"; the callee just won't be able to modify the
struct, even if the caller can.


The problem is when you start declaring function pointers in various ops vectors.

Consider:

void (*foo)(const struct pci_dev *)
void (*bar)(struct pci_dev *)

foo and bar are not type compatible, and you will get compiler warnings if you use one where the other is expected.

So the question is: Are we ever going to the address of any of the functions that are being modified? If so, we have created a problem.

Similar reasoning applies to of_irq_map_pci().

So I'm fine with this.  You sent it to Grant, so I'll assume he'll
merge it unless I hear otherwise.

Acked-by: Bjorn Helgaas<bhelgaas@google.com>




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