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SubjectRe: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only
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On 11/16/21 10:20 AM, Rob Herring wrote:
> +Marc Z
>
> On Mon, Nov 15, 2021 at 8:39 AM Andy Shevchenko
> <andriy.shevchenko@linux.intel.com> wrote:
>>
>> On Mon, Nov 15, 2021 at 04:14:21PM +0200, Andy Shevchenko wrote:
>>> On Mon, Nov 15, 2021 at 4:01 PM Robin Murphy <robin.murphy@arm.com> wrote:
>>>> On 2021-11-15 11:20, Andy Shevchenko wrote:
>>>>> Use BIT() as __GENMASK() is for internal use only. The rationale
>>>>> of switching to BIT() is to provide better generated code. The
>>>>> GENMASK() against non-constant numbers may produce an ugly assembler
>>>>> code. On contrary the BIT() is simply converted to corresponding shift
>>>>> operation.
>>>>
>>>> FWIW, If you care about code quality and want the compiler to do the
>>>> obvious thing, why not specify it as the obvious thing:
>>>>
>>>> u32 val = ~0 << msi->legacy_shift;
>>>
>>> Obvious and buggy (from the C standard point of view)? :-)
>>
>> Forgot to mention that BIT() is also makes it easy to avoid such mistake.
>>
>>>> Personally I don't think that abusing BIT() in the context of setting
>>>> multiple bits is any better than abusing __GENMASK()...
>>>
>>> No, BIT() is not abused here, but __GENMASK().
>>>
>>> After all it's up to you, folks, consider that as a bug report.
>
> Couldn't we get rid of legacy_shift entirely if the legacy case sets
> up 'hwirq' as 24-31 rather than 0-7? Though the data for the MSI msg
> uses the hwirq.

I personally find it clearer and easier to reason about with the current
code though I suppose that with an appropriate xlate method we could
sort of set up the hwirq the way we want them to be to avoid any
shifting in brcm_pcie_msi_isr().
--
Florian
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