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

From: Lukas Wunner <lukas@wunner.de>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>,
	Mark Brown <broonie@kernel.org>,
	"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
	<linux-arm-kernel@lists.infradead.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>,
	Scott Branden <sbranden@broadcom.com>,
	Ray Jui <rjui@broadcom.com>,
	linux-kernel@vger.kernel.org,
	"open list:SPI SUBSYSTEM" <linux-spi@vger.kernel.org>,
	Rob Herring <robh+dt@kernel.org>,
	"maintainer:BROADCOM BCM281XX/BCM11XXX/BCM216XX ARM ARCHITE..." 
	<bcm-kernel-feedback-list@broadcom.com>,
	"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
	<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
Date: Mon, 8 Jun 2020 13:41:48 +0200	[thread overview]
Message-ID: <20200608114148.4bau4mdcvwgf25ut@wunner.de> (raw)
In-Reply-To: <a6f158e3-af51-01d9-331c-4bc8b6847abb@arm.com>

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