linux-kernel - Re: [PATCH v4] iio: adc: xilinx-xadc: Push interrupts into hardirq context

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Date:	Wed, 12 Aug 2015 17:17:37 +0200
From:	Lars-Peter Clausen <lars@...afoo.de>
To:	Xander Huff <xander.huff@...com>, jic23@...nel.org,
	bigeasy@...utronix.de
CC:	knaack.h@....de, pmeerw@...erw.net, michal.simek@...inx.com,
	soren.brinkmann@...inx.com, linux-iio@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	joe.hershberger@...com, joshc@...com, nathan.sullivan@...com,
	jaeden.amero@...com
Subject: Re: [PATCH v4] iio: adc: xilinx-xadc: Push interrupts into hardirq
 context

On 08/12/2015 01:00 AM, Xander Huff wrote:
> The driver currently registers a pair of irq handlers using
> request_threaded_irq(), however the synchronization mechanism between the
> hardirq and the threadedirq handler is a regular spinlock.
> 
> Unfortunately, this breaks PREEMPT_RT builds, where a spinlock can sleep,
> and is thus not able to be acquired from a hardirq handler. This patch gets
> rid of the threaded handler and pushes all interrupt handling into the
> hardirq context, and uses request_irq().
> 
> To validate that this change has no impact on RT performance, here are
> cyclictest values with no processes running:
> 
> $ sudo cyclictest -S -m -p 98
> # /dev/cpu_dma_latency set to 0us
> policy: fifo: loadavg: 0.00 0.01 0.05 1/174 2539
> T: 0 ( 1405) P:98 I:1000 C:167010520 Min: 9 Act: 12 Avg: 12 Max: 75
> T: 1 ( 1862) P:98 I:1500 C:111340339 Min: 9 Act: 12 Avg: 12 Max: 73
> 
> Then, all xadc raw handles were accessed in a continuous loop via
> /sys/bus/iio/devices/iio:device0:
> 
> $ sudo cyclictest -S -m -p 98
> # /dev/cpu_dma_latency set to 0us
> policy: fifo: loadavg: 7.84 7.70 7.63 3/182 4260
> T: 0 ( 2559) P:98 I:1000 C:241557018 Min: 11 Act: 18 Avg: 21 Max: 74
> T: 1 ( 2560) P:98 I:1500 C:161038006 Min: 10 Act: 21 Avg: 20 Max: 73
> 
> Signed-off-by: Xander Huff <xander.huff@...com>

Looks good, thanks.

Acked-by: Lars-Peter Clausen <lars@...afoo.de>
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