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

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Date:   Mon, 8 Jun 2020 13:41:48 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Mark Brown <broonie@...nel.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>,
        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:
> And all in code that has at least one obvious inefficiency left on
> the table either way.

Care to submit a patch to overcome that inefficiency?

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

The thread came about because it can be determined at compile time
whether the interrupt is going to be shared:

On the BCM2835 (Raspberry Pi 1), CONFIG_ARCH_MULTI_V6 is set and this
SoC doesn't have multiple bcm2835-spi instances, so no shared interrupt.

The question is how to discern BCM2836/BCM2837 (Raspberry Pi 2/3), which
do not have multiple instances, and BCM2711 (Raspberry Pi 4) which does.

The Raspberry Pi Foundation compiles BCM2711 kernels with CONFIG_ARM_LPAE=y,
but Florian considered that kludgy as a discriminator and opted for
runtime-detection via the compatible string instead.  If you've got
a better idea please come forward.

Is "optimize shared IRQ support away if IS_ENABLED(CONFIG_ARCH_MULTI_V6),
else leave it in" the best we can do?

Thanks,

Lukas