Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only

From: Rob Herring <robh@kernel.org>
To: Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Marc Zyngier <maz@kernel.org>
Cc: "Robin Murphy" <robin.murphy@arm.com>,
	bcm-kernel-feedback-list <bcm-kernel-feedback-list@broadcom.com>,
	linux-rpi-kernel <linux-rpi-kernel@lists.infradead.org>,
	"linux-arm Mailing List" <linux-arm-kernel@lists.infradead.org>,
	linux-pci <linux-pci@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Jim Quinlan" <jim2101024@gmail.com>,
	"Nicolas Saenz Julienne" <nsaenz@kernel.org>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	"Lorenzo Pieralisi" <lorenzo.pieralisi@arm.com>,
	"Krzysztof Wilczyński" <kw@linux.com>,
	"Bjorn Helgaas" <bhelgaas@google.com>
Subject: Re: [PATCH v2 1/1] PCI: brcmstb: Use BIT() as __GENMASK() is for internal use only
Date: Tue, 16 Nov 2021 12:20:22 -0600	[thread overview]
Message-ID: <CAL_JsqJndi-gmenSpPtMVfsb3SrA=w+YBsSh3GigfgXC3rYDeQ@mail.gmail.com> (raw)
In-Reply-To: <YZJxG7JFAfIqr1/f@smile.fi.intel.com>

+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.

Rob