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SubjectRe: [PATCH 4/5] pci: handled return value of platform_get_irq correctly
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On 12/03/2020 15:11, Bjorn Helgaas wrote:

> [+cc another Marc]

Doh! I should indeed have CCed maz and tglx.

> On Thu, Mar 12, 2020 at 10:53:06AM +0100, Marc Gonzalez wrote:
>
>> On 11/03/2020 20:19, Aman Sharma wrote:
>>
>>> diff --git a/drivers/pci/controller/pcie-tango.c b/drivers/pci/controller/pcie-tango.c
>>> index 21a208da3f59..18c2c4313eb5 100644
>>> --- a/drivers/pci/controller/pcie-tango.c
>>> +++ b/drivers/pci/controller/pcie-tango.c
>>> @@ -273,9 +273,9 @@ static int tango_pcie_probe(struct platform_device *pdev)
>>> writel_relaxed(0, pcie->base + SMP8759_ENABLE + offset);
>>>
>>> virq = platform_get_irq(pdev, 1);
>>> - if (virq <= 0) {
>>> + if (virq < 0) {
>>> dev_err(dev, "Failed to map IRQ\n");
>>> - return -ENXIO;
>>> + return virq;
>>> }
>>>
>>> irq_dom = irq_domain_create_linear(fwnode, MSI_MAX, &dom_ops, pcie);
>>
>> Weee, here we go again :-)
>>
>> https://patchwork.kernel.org/patch/11066455/
>> https://patchwork.kernel.org/patch/10006651/
>>
>> Last time around, my understanding was that, going forward,
>> the best solution was:
>>
>> virq = platform_get_irq(...)
>> if (virq <= 0)
>> return virq ? : -ENODEV;
>>
>> i.e. map 0 to -ENODEV, pass other errors as-is, remove the dev_err
>>
>> @Bjorn/Lorenzo did you have a change of heart?
>
> Yes. In 10006651 (Oct 20, 2017), I thought:
>
> irq = platform_get_irq(pdev, 0);
> if (irq <= 0)
> return -ENODEV;
>
> was fine. In 11066455 (Aug 7, 2019), I said I thought I was wrong and
> that:
>
> platform_get_irq() is a generic interface and we have to be able to
> interpret return values consistently. The overwhelming consensus
> among platform_get_irq() callers is to treat "irq < 0" as an error,
> and I think we should follow suit.
> ...
> I think the best pattern is:
>
> irq = platform_get_irq(pdev, i);
> if (irq < 0)
> return irq;
>
> I still think what I said in 2019 is the right approach. I do see
> your comment in 10006651 about this pattern:
>
> if (virq <= 0)
> return virq ? : -ENODEV;
>
> but IMHO it's too complicated for general use. Admittedly, it's not
> *very* complicated, but it's a relatively unusual C idiom and I
> stumble over it every time I see it.

FTR, omitting the middle operand is a GNU extension.
https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html
The valid C idiom would be virq ? virq : -ENODEV

> If 0 is a special case I think
> it should be mapped to a negative error in arch-specific code, which I
> think is what Linus T suggested in [1].

Lorenzo, being both PCI maintainer and ARM employee should be in a
good position to change the arch-specific code for arm and arm64?

> I think there's still a large consensus that "irq < 0" is the error
> case. In the tree today we have about 1400 callers of
> platform_get_irq() and platform_get_irq_byname() [2]. Of those,
> almost 900 check for "irq < 0" [3], while only about 150 check for
> "irq <= 0" [4] and about 15 use some variant of a "irq ? : -ENODEV"
> pattern.
>
> The bottom line is that in drivers/pci, I'd like to see either a
> single style or a compelling argument for why some checks should be
> "irq < 0" and others should be "irq <= 0".
>
> [1] https://yarchive.net/comp/linux/zero.html
> [2] $ git grep "=.*platform_get_irq" | wc -l
> 1422
> [3] $ git grep -A4 "=.*platform_get_irq" | grep "<\s*0" | wc -l
> 894
> [4] $ git grep -A4 "=.*platform_get_irq" | grep "<=\s*0" | wc -l
> 151
> [5] $ git grep -A4 "=.*platform_get_irq" | grep "return.*?.*:.*;" | wc -l
> 15

Interesting stats, thanks.

Regards.

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