Re: [PATCH] OF: PCI: const usage needed by MIPS
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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@xxxxxxxxxxx> 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 DaneyHi, 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@xxxxxxxxxx>
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