linux-kernel - Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support

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Date:   Mon, 8 Jun 2020 12:28:40 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Florian Fainelli <f.fainelli@...il.com>, lukas@...ner.de,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Scott Branden <sbranden@...adcom.com>,
        Ray Jui <rjui@...adcom.com>, linux-kernel@...r.kernel.org,
        "open list:SPI SUBSYSTEM" <linux-spi@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "maintainer:BROADCOM BCM281XX/BCM11XXX/BCM216XX ARM ARCHITE..." 
        <bcm-kernel-feedback-list@...adcom.com>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>,
        Martin Sperl <kernel@...tin.sperl.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Subject: Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support

On Mon, Jun 08, 2020 at 12:11:11PM +0100, Robin Murphy wrote:

> Again, 2 cycles. The overhead of a static key alone is at least 50% of that.
> And that's not even considering whether the change in code layout caused by
> doubling up the IRQ handler might affect I-cache or branch predictor
> behaviour, where a single miss stands to more than wipe out any perceived
> saving. And all in code that has at least one obvious inefficiency left on
> the table either way.

> This thread truly epitomises Knuth's "premature optimisation" quote... ;)

In fairness the main reason this driver is so heavily tuned already (and
has lead to some really nice improvements in the core) is that there are
a number of users hitting 100% CPU utilization driving SPI devices on
some of the older RPi hardware, IIRC around IIO type applications
mostly.  I do tend to agree that this particular optimization is a bit
marginal but there has been a lot of effort put into this.

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