linux-kernel - Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as _

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Message-ID: <71a90592-99bb-13e1-a671-eb19c2dad3da@broadcom.com>
Date:   Tue, 16 Nov 2021 12:41:28 -0800
From:   Florian Fainelli <florian.fainelli@...adcom.com>
To:     Rob Herring <robh@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Marc Zyngier <maz@...nel.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        linux-rpi-kernel <linux-rpi-kernel@...ts.infradead.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jim Quinlan <jim2101024@...il.com>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for
 internal use only

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@...ux.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@....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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