From: Mark Brown <broonie@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>,
lukas@wunner.de,
"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 12:28:40 +0100 [thread overview]
Message-ID: <20200608112840.GC4593@sirena.org.uk> (raw)
In-Reply-To: <a6f158e3-af51-01d9-331c-4bc8b6847abb@arm.com>
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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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WARNING: multiple messages have this Message-ID (diff)
From: Mark Brown <broonie@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
<devicetree@vger.kernel.org>,
Florian Fainelli <f.fainelli@gmail.com>,
Scott Branden <sbranden@broadcom.com>,
Ray Jui <rjui@broadcom.com>,
linux-kernel@vger.kernel.org, Rob Herring <robh+dt@kernel.org>,
"open list:SPI SUBSYSTEM" <linux-spi@vger.kernel.org>,
Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
lukas@wunner.de,
"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>,
"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support
Date: Mon, 8 Jun 2020 12:28:40 +0100 [thread overview]
Message-ID: <20200608112840.GC4593@sirena.org.uk> (raw)
In-Reply-To: <a6f158e3-af51-01d9-331c-4bc8b6847abb@arm.com>
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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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next prev parent reply other threads:[~2020-06-08 11:28 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-04 21:28 [PATCH v2] spi: bcm2835: Enable shared interrupt support Florian Fainelli
2020-06-04 21:28 ` Florian Fainelli
2020-06-05 8:46 ` Nicolas Saenz Julienne
2020-06-05 8:46 ` Nicolas Saenz Julienne
2020-06-05 10:52 ` Mark Brown
2020-06-05 10:52 ` Mark Brown
2020-06-05 10:58 ` Nicolas Saenz Julienne
2020-06-05 10:58 ` Nicolas Saenz Julienne
2020-06-05 10:51 ` Lukas Wunner
2020-06-05 11:14 ` Mark Brown
2020-06-05 11:14 ` Mark Brown
2020-06-05 12:20 ` Mark Brown
2020-06-05 12:20 ` Mark Brown
2020-06-05 11:34 ` Robin Murphy
2020-06-05 11:34 ` Robin Murphy
2020-06-05 13:20 ` Mark Brown
2020-06-05 13:20 ` Mark Brown
2020-06-05 13:46 ` Robin Murphy
2020-06-05 13:46 ` Robin Murphy
2020-06-05 14:41 ` Robin Murphy
2020-06-05 14:41 ` Robin Murphy
2020-06-05 15:27 ` Mark Brown
2020-06-05 15:27 ` Mark Brown
2020-06-05 22:04 ` Florian Fainelli
2020-06-05 22:04 ` Florian Fainelli
2020-06-08 11:11 ` Robin Murphy
2020-06-08 11:11 ` Robin Murphy
2020-06-08 11:28 ` Mark Brown [this message]
2020-06-08 11:28 ` Mark Brown
2020-06-15 16:34 ` Florian Fainelli
2020-06-15 16:34 ` Florian Fainelli
2020-06-15 17:00 ` Mark Brown
2020-06-15 17:00 ` Mark Brown
2020-06-15 17:04 ` Florian Fainelli
2020-06-15 17:04 ` Florian Fainelli
2020-06-15 17:30 ` Mark Brown
2020-06-15 17:30 ` Mark Brown
2020-06-15 17:31 ` Robin Murphy
2020-06-15 17:31 ` Robin Murphy
2020-06-15 19:26 ` Mark Brown
2020-06-15 19:26 ` Mark Brown
2020-06-08 11:41 ` Lukas Wunner
2020-06-15 19:09 ` Robin Murphy
2020-06-15 19:09 ` Robin Murphy
2020-06-15 19:42 ` Florian Fainelli
2020-06-15 19:42 ` Florian Fainelli
2020-06-15 20:48 ` Mark Brown
2020-06-15 20:48 ` Mark Brown
-- strict thread matches above, loose matches on Subject: below --
2022-07-19 10:53 [PATCH v2] spi: bcm2835: enable " Marc Kleine-Budde
2022-07-26 11:17 ` Mark Brown
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