From: Florian Fainelli <f.fainelli@gmail.com>
To: Robin Murphy <robin.murphy@arm.com>, Lukas Wunner <lukas@wunner.de>
Cc: 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, 15 Jun 2020 12:42:50 -0700 [thread overview]
Message-ID: <2e4a6c4b-857f-1000-e7fd-327b0800fb97@gmail.com> (raw)
In-Reply-To: <3d4fd3f9-9bde-90a8-bef5-9fc97cc9b363@arm.com>
On 6/15/2020 12:09 PM, Robin Murphy wrote:
> On 2020-06-08 12:41, Lukas Wunner wrote:
>> 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?
>
> I'll have a quick go, but without any way to measure performance impact
> (or even test for correctness) I don't fancy going too deep based purely
> on disassembly and ARM11 cycle timings.
>
>>> 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:
>
> ...which is exactly my point - "because it can be" is anything but proof
> that avoiding a trivial check makes enough measurable difference to
> justify putting in the effort to do so.
>
>> 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.
>
> Hmm, how much relative importance does that have? On a 700MHz ARM11 it's
> obviously desirable to spend as little time in the IRQ handler as
> possible in order to have time left to do anything else, but on the
> other SoCs even if the IRQ remains permanently asserted it can still
> only consume 25% of the available CPU capacity, at which point the
> impact of 2-3 cycles either way at 1GHz+ seems pretty much immeasurable.
>
>> 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?
>
> In all honesty I'm starting to think it seriously might be :)
Or how about this: we slightly re-structure the interrupt handler such
that all possible interrupt conditions are explicitly handled and
terminate with a return IRQ_HANDLED, and those which are not, including
in the case of a "spurious" (because the interrupt was triggered for
another SPI controller instance), then we finish the function with
IRQ_NONE. This would not impact the performance for the BCM2835/36/37
which would still have a single controller/single interrupt line to handle.
--
Florian
next prev parent reply other threads:[~2020-06-15 19:42 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-04 21:28 [PATCH v2] spi: bcm2835: Enable shared interrupt support Florian Fainelli
2020-06-05 8:46 ` Nicolas Saenz Julienne
2020-06-05 10:52 ` Mark Brown
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 12:20 ` Mark Brown
2020-06-05 11:34 ` Robin Murphy
2020-06-05 13:20 ` Mark Brown
2020-06-05 13:46 ` Robin Murphy
2020-06-05 14:41 ` Robin Murphy
2020-06-05 15:27 ` Mark Brown
2020-06-05 22:04 ` Florian Fainelli
2020-06-08 11:11 ` Robin Murphy
2020-06-08 11:28 ` Mark Brown
2020-06-15 16:34 ` Florian Fainelli
2020-06-15 17:00 ` Mark Brown
2020-06-15 17:04 ` Florian Fainelli
2020-06-15 17:30 ` Mark Brown
2020-06-15 17:31 ` Robin Murphy
2020-06-15 19:26 ` Mark Brown
2020-06-08 11:41 ` Lukas Wunner
2020-06-15 19:09 ` Robin Murphy
2020-06-15 19:42 ` Florian Fainelli [this message]
2020-06-15 20:48 ` Mark Brown
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2e4a6c4b-857f-1000-e7fd-327b0800fb97@gmail.com \
--to=f.fainelli@gmail.com \
--cc=bcm-kernel-feedback-list@broadcom.com \
--cc=broonie@kernel.org \
--cc=devicetree@vger.kernel.org \
--cc=kernel@martin.sperl.org \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rpi-kernel@lists.infradead.org \
--cc=linux-spi@vger.kernel.org \
--cc=lukas@wunner.de \
--cc=nsaenzjulienne@suse.de \
--cc=rjui@broadcom.com \
--cc=robh+dt@kernel.org \
--cc=robin.murphy@arm.com \
--cc=sbranden@broadcom.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).