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

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Date:   Tue, 16 Nov 2021 12:20:22 -0600
From:   Rob Herring <robh@...nel.org>
To:     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

+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.

Rob