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Date:   Fri, 5 Jun 2020 16:27:39 +0100
From:   Mark Brown <broonie@kernel.org>
To:     Robin Murphy <robin.murphy@arm.com>
Cc:     "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>,
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        <devicetree@vger.kernel.org>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Scott Branden <sbranden@broadcom.com>, lukas@wunner.de,
        Ray Jui <rjui@broadcom.com>, linux-kernel@vger.kernel.org,
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        Rob Herring <robh+dt@kernel.org>,
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        <linux-rpi-kernel@lists.infradead.org>,
        Martin Sperl <kernel@martin.sperl.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Subject: Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support
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On Fri, Jun 05, 2020 at 03:41:27PM +0100, Robin Murphy wrote:

> Ha, and in fact having checked a build out of curiosity, this patch as-is
> actually stands to make things considerably worse. At least with GCC 8.3 and
> bcm2835_defconfig, bcm2835_spi_interrupt_common() doesn't get inlined, which
> means bcm2835_spi_interrupt() pushes/pops a stack frame and makes an
> out-of-line call to bcm2835_spi_interrupt_common(), resulting in massively
> *more* work than the extra two instructions of simply inlining the test.

Whichever compiler I was using (clang-11 probably) did manage to inline
the tail call so it really was the two instructions but yeah, in general
this approach is going to be fragile.

> So yes, the overhead of inlining the test vs. the alternative is indeed
> non-zero. It's just also negative :D

And variable!

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