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

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Message-ID: <94D0CD8314A33A4D9D801C0FE68B4029593C5E1F@G9W0745.americas.hpqcorp.net>
Date:	Tue, 25 Nov 2014 00:53:19 +0000
From:	"Elliott, Robert (Server Storage)" <Elliott@...com>
To:	Mitchel Humpherys <mitchelh@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"iommu@...ts.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>
CC:	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



> -----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.

---
Rob Elliott    HP Server Storage



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