| Message ID | d25a332bbb792f4c8d74f7e54bf9ca1d706979d9.1670495642.git.geert+renesas@glider.be (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | iopoll: Call cpu_relax() in busy loops | expand |
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; \ })
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@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(+)