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

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Date:   Wed, 17 Nov 2021 12:37:20 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Rob Herring <robh@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Marc Zyngier <maz@...nel.org>,
        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>,
        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 Wed, Nov 17, 2021 at 12:42 AM Florian Fainelli <f.fainelli@...il.com> wrote:
>
> On 11/16/21 12:41 PM, Florian Fainelli wrote:
> > 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().
>
> Something like the following maybe? Completely untested as I don't
> believe I have a device with that legacy controller available at the moment:

Since it gets rid of __GENMASK (ab)use, I'm fine to see it applied at
some point.

-- 
With Best Regards,
Andy Shevchenko