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

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Date:   Mon, 15 Nov 2021 16:39:23 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     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>,
        Rob Herring <robh@...nel.org>,
        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 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.

-- 
With Best Regards,
Andy Shevchenko