[PATCH] iopoll: Call cpu_relax() in busy loops

[Geert Uytterhoeven]

It is considered good practice to call cpu_relax() in busy loops, see
Documentation/process/volatile-considered-harmful.rst.  This can not
only lower CPU power consumption or yield to a hyperthreaded twin
processor, but also allows an architecture to mitigate hardware issues
(e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the
architecture-specific cpu_relax() implementation.

As the iopoll helpers lack calls to cpu_relax(), people are sometimes
reluctant to use them, and may fall back to open-coded polling loops
(including cpu_relax() calls) instead.

Fix this by adding calls to cpu_relax() to the iopoll helpers:
  - For the non-atomic case, it is sufficient to call cpu_relax() in
    case of a zero sleep-between-reads value, as a call to
    usleep_range() is a safe barrier otherwise.
  - For the atomic case, cpu_relax() must be called regardless of the
    sleep-between-reads value, as there is no guarantee all
    architecture-specific implementations of udelay() handle this.

Signed-off-by: Geert Uytterhoeven <geert+renesas at glider.be>
---
This has been discussed before, but I am not aware of any patches moving
forward:
  - "Re: [PATCH 6/7] clk: renesas: rcar-gen3: Add custom clock for PLLs"
    https://lore.kernel.org/all/CAMuHMdWUEhs=nwP+a0vO2jOzkq-7FEOqcJ+SsxAGNXX1PQ2KMA@mail.gmail.com/
  - "Re: [PATCH v2] clk: samsung: Prevent potential endless loop in the PLL set_rate ops"
    https://lore.kernel.org/all/20200811164628.GA7958@kozik-lap
---
 include/linux/iopoll.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/iopoll.h b/include/linux/iopoll.h
index 2c8860e406bd8cae..73132721d1891a2e 100644
--- a/include/linux/iopoll.h
+++ b/include/linux/iopoll.h
@@ -53,6 +53,8 @@
 		} \
 		if (__sleep_us) \
 			usleep_range((__sleep_us >> 2) + 1, __sleep_us); \
+		else \
+			cpu_relax(); \
 	} \
 	(cond) ? 0 : -ETIMEDOUT; \
 })
@@ -95,6 +97,7 @@
 		} \
 		if (__delay_us) \
 			udelay(__delay_us); \
+		cpu_relax(); \
 	} \
 	(cond) ? 0 : -ETIMEDOUT; \
 })
-- 
2.25.1