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

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Message-ID: <20200605132037.GF5413@sirena.org.uk>
Date:   Fri, 5 Jun 2020 14:20:37 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org,
        "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>, lukas@...ner.de,
        Ray Jui <rjui@...adcom.com>, Rob Herring <robh+dt@...nel.org>,
        "open list:SPI SUBSYSTEM" <linux-spi@...r.kernel.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 Fri, Jun 05, 2020 at 12:34:36PM +0100, Robin Murphy wrote:
> On 2020-06-04 22:28, Florian Fainelli wrote:

> > For the BCM2835 case which is deemed performance critical, we would like
> > to continue using an interrupt handler which does not have the extra
> > comparison on BCM2835_SPI_CS_INTR.

> FWIW, if I'm reading the patch correctly, then with sensible codegen that
> "overhead" should amount to a bit test on a live register plus a not-taken
> conditional branch - according to the 1176 TRM that should add up to a
> whopping 2 cycles. If that's really significant then I'd have to wonder
> whether you want to be at the mercy of the whole generic IRQ stack at all,
> and should perhaps consider using FIQ instead.

Yes, and indeed the compiler does seem to manage that.  It *is* non-zero
overhead though.

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