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

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Message-ID: <bb9dbf11-9e33-df60-f5ae-f7fdfe8458b4@gmail.com>
Date:   Mon, 15 Jun 2020 09:34:58 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Mark Brown <broonie@...nel.org>,
        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 6/8/2020 4:28 AM, Mark Brown wrote:
> 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.

OK, so this has been dropped for spi/for-next right? How do we move from
there?
-- 
Florian