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

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Date:   Fri, 5 Jun 2020 15:41:27 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Mark Brown <broonie@...nel.org>
Cc:     "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>,
        Florian Fainelli <f.fainelli@...il.com>,
        Scott Branden <sbranden@...adcom.com>, lukas@...ner.de,
        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 2020-06-05 14:46, Robin Murphy wrote:
> On 2020-06-05 14:20, Mark Brown wrote:
>> 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.
> 
> True, but so's the existing level of pointer-chasing indirection that 
> with some straightforward refactoring could be taken right out of the 
> critical path and confined to just the conditional complete() call. 
> That's the kind of thing leaving me unconvinced that this is code where 
> every single cycle counts ;)

Ha, and in fact having checked a build out of curiosity, this patch 
as-is actually stands to make things considerably worse. At least with 
GCC 8.3 and bcm2835_defconfig, bcm2835_spi_interrupt_common() doesn't 
get inlined, which means bcm2835_spi_interrupt() pushes/pops a stack 
frame and makes an out-of-line call to bcm2835_spi_interrupt_common(), 
resulting in massively *more* work than the extra two instructions of 
simply inlining the test.

So yes, the overhead of inlining the test vs. the alternative is indeed 
non-zero. It's just also negative :D

Robin.