linux-kernel - Re: [PATCH RESEND v8] iopoll: Introduce memory-mapped IO polling macros

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Message-ID: <vnkwy4r0gjne.fsf@mitchelh-linux.qualcomm.com>
Date:	Mon, 24 Nov 2014 17:21:09 -0800
From:	Mitchel Humpherys <mitchelh@...eaurora.org>
To:	"Elliott\, Robert \(Server Storage\)" <Elliott@...com>
Cc:	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel\@lists.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"iommu\@lists.linux-foundation.org" 
	<iommu@...ts.linux-foundation.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Wagantall <mattw@...eaurora.org>,
	"Don Brace \(PMC\)" <Don.Brace@...s.com>,
	"Scales\, Webb" <webb.scales@...com>
Subject: Re: [PATCH RESEND v8] iopoll: Introduce memory-mapped IO polling macros

On Mon, Nov 24 2014 at 04:53:19 PM, "Elliott, Robert (Server Storage)" <Elliott@...com> wrote:
>> -----Original Message-----
>> From: linux-kernel-owner@...r.kernel.org [mailto:linux-kernel-
>> owner@...r.kernel.org] On Behalf Of Mitchel Humpherys
>> Sent: Monday, 24 November, 2014 2:15 PM
> ...
>> From: Matt Wagantall <mattw@...eaurora.org>
>> 
>> It is sometimes necessary to poll a memory-mapped register until its value
>> satisfies some condition. Introduce a family of convenience macros that do
>> this. Tight-looping, sleeping, and timing out can all be accomplished
>> using these macros.
>> 
> ...
>> +#define readx_poll_timeout(op, addr, val, cond, sleep_us, timeout_us)	\
>> +({ \
>> +	ktime_t timeout = ktime_add_us(ktime_get(), timeout_us); \
>> +	might_sleep_if(sleep_us); \
>> +	for (;;) { \
>> +		(val) = op(addr); \
>> +		if (cond) \
>> +			break; \
>> +		if (timeout_us && ktime_compare(ktime_get(), timeout) > 0) {
>> \
>> +			(val) = op(addr); \
>> +			break; \
>> +		} \
>> +		if (sleep_us) \
>> +			usleep_range((sleep_us >> 2) + 1, sleep_us); \
>
> The hpsa SCSI driver used to use usleep_range in a loop like 
> that, but we found that it caused scheduler problems during
> boots because it uses TASK_UNINTERRUPTIBLE:
> [    9.260668] [sched_delayed] sched: RT throttling activated
>
> msleep() worked much better.

Hmm, maybe you were just sleeping for too long?  According to
Documentation/timers/timers-howto.txt, usleep_range is what should be
used for non-atomic sleeps in the range [10us, 20ms].  Plus we need
microsecond granularity anyways, so msleep wouldn't cut it.

If there are any potential users of these macros that would want to
sleep for more than 20ms I guess we could add a special case here to use
msleep when sleep_us exceeds 20,000 or so.


-Mitch

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Qualcomm Innovation Center, Inc.
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