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[83.57.172.194]) by smtp.gmail.com with ESMTPSA id b200sm30290701wmb.10.2021.01.03.12.51.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 03 Jan 2021 12:51:55 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=IdHQJwCqOdYJyKNmtS/fZyUmMRQdfAB5wRMdzdWKipA=; b=HwYHJYDTTT9fz3lggP7dIYTMzsuwzpdQPoIhTRpZt8Odep4aqgeq1M1vCcSGk6BMxN wjpPuYtsgM4u8gSYof+X0cYw045wEGSesiyNvidjKXS1kRLgKCOQXpnVtQUEulVs3d9U yS+FuOoPFm5QtsSTH6m3sKw1ic5mNmv+ngB/d5tPgVctJIEIGubmHvKeuHDkWbPVm4xG MPIVV+E3jZZ/eVjoO+BzDzMo++p+yWgZo8sDEHtQeUpjq8MQrkhQ1hEArvje38I7v3uO 3m3XB27y4gjsgMJcTGmxXI3DYxXwqqpsTvoAkj+NRkIHDJKkmGdeuAl0FluyrlFXNgO5 a5Vw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:from:to:cc:subject:date:message-id :in-reply-to:references:mime-version:content-transfer-encoding; bh=IdHQJwCqOdYJyKNmtS/fZyUmMRQdfAB5wRMdzdWKipA=; b=NLHCpRc/BLe2cJ+h5iNBUY19OsdJncyeXzwBPcdBMf9CYAqb2i6ULNbJ7tN9gQC4Ro nJfzRJClTVabTV1xLP66M83tjbceu603x98l8Ur9sabijH4ehmXslbgrLePo51h5SMrh HmY+VUdrLzS3zpWbHsj7FAan8M0yD3KmOjzyhwYZBKk7WLWAzqeKmAMR8fjOpSGtp9P1 amb01tFXumQbyzIryxuTc55tNC/5ZzYVmhaRRy07OkKxHFKggCiSR4YmrXD5upIrKopN mKd1JJFbDArORRqPFMwFvA0KOji8VvV9hXJDsyqUmb+MqFXbY/Y+aCIr5xCqSC3k+u4H jv1A== X-Gm-Message-State: AOAM531PYaLrmUlnxeXe6mJVAgCt+qywrDjnr0hVsvTDK10mZVaqElfB cbR/jkj23ao8lPOT4l3GKyc= X-Google-Smtp-Source: ABdhPJy2/sxQWC+xPi4mr8rRCS85dS9tT/eZzjgYUqI5H2zLe8Uge0RuVlVV21/Ktg5AeHIvhFCewg== X-Received: by 2002:adf:c387:: with SMTP id p7mr76731410wrf.95.1609707115943; Sun, 03 Jan 2021 12:51:55 -0800 (PST) Sender: =?UTF-8?Q?Philippe_Mathieu=2DDaud=C3=A9?= From: =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= To: qemu-devel@nongnu.org Cc: Aurelien Jarno , Huacai Chen , Jiaxun Yang , Peter Maydell , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , Luc Michel , Richard Henderson Subject: [PULL 19/35] clock: Introduce clock_ticks_to_ns() Date: Sun, 3 Jan 2021 21:50:05 +0100 Message-Id: <20210103205021.2837760-20-f4bug@amsat.org> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20210103205021.2837760-1-f4bug@amsat.org> References: <20210103205021.2837760-1-f4bug@amsat.org> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @gmail.com) From: Peter Maydell The clock_get_ns() API claims to return the period of a clock in nanoseconds. Unfortunately since it returns an integer and a clock's period is represented in units of 2^-32 nanoseconds, the result is often an approximation, and calculating a clock expiry deadline by multiplying clock_get_ns() by a number-of-ticks is unacceptably inaccurate. Introduce a new API clock_ticks_to_ns() which returns the number of nanoseconds it takes the clock to make a given number of ticks. This function can do the complete calculation internally and will thus give a more accurate result. Signed-off-by: Peter Maydell Tested-by: Philippe Mathieu-Daud=C3=A9 Reviewed-by: Luc Michel Reviewed-by: Richard Henderson Reviewed-by: Philippe Mathieu-Daud=C3=A9 Message-Id: <20201215150929.30311-2-peter.maydell@linaro.org> Signed-off-by: Philippe Mathieu-Daud=C3=A9 --- docs/devel/clocks.rst | 29 +++++++++++++++++++++++++++++ include/hw/clock.h | 41 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 70 insertions(+) diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst index e5da28e2111..c2e70e64db1 100644 --- a/docs/devel/clocks.rst +++ b/docs/devel/clocks.rst @@ -258,6 +258,35 @@ Here is an example: clock_get_ns(dev->my_clk_input)); } =20 +Calculating expiry deadlines +---------------------------- + +A commonly required operation for a clock is to calculate how long +it will take for the clock to tick N times; this can then be used +to set a timer expiry deadline. Use the function ``clock_ticks_to_ns()``, +which takes an unsigned 64-bit count of ticks and returns the length +of time in nanoseconds required for the clock to tick that many times. + +It is important not to try to calculate expiry deadlines using a +shortcut like multiplying a "period of clock in nanoseconds" value +by the tick count, because clocks can have periods which are not a +whole number of nanoseconds, and the accumulated error in the +multiplication can be significant. + +For a clock with a very long period and a large number of ticks, +the result of this function could in theory be too large to fit in +a 64-bit value. To avoid overflow in this case, ``clock_ticks_to_ns()`` +saturates the result to INT64_MAX (because this is the largest valid +input to the QEMUTimer APIs). Since INT64_MAX nanoseconds is almost +300 years, anything with an expiry later than that is in the "will +never happen" category. Callers of ``clock_ticks_to_ns()`` should +therefore generally not special-case the possibility of a saturated +result but just allow the timer to be set to that far-future value. +(If you are performing further calculations on the returned value +rather than simply passing it to a QEMUTimer function like +``timer_mod_ns()`` then you should be careful to avoid overflow +in those calculations, of course.) + Changing a clock period ----------------------- =20 diff --git a/include/hw/clock.h b/include/hw/clock.h index 81bcf3e505a..b5fff6ded83 100644 --- a/include/hw/clock.h +++ b/include/hw/clock.h @@ -16,6 +16,8 @@ =20 #include "qom/object.h" #include "qemu/queue.h" +#include "qemu/host-utils.h" +#include "qemu/bitops.h" =20 #define TYPE_CLOCK "clock" OBJECT_DECLARE_SIMPLE_TYPE(Clock, CLOCK) @@ -218,6 +220,45 @@ static inline unsigned clock_get_ns(Clock *clk) return CLOCK_PERIOD_TO_NS(clock_get(clk)); } =20 +/** + * clock_ticks_to_ns: + * @clk: the clock to query + * @ticks: number of ticks + * + * Returns the length of time in nanoseconds for this clock + * to tick @ticks times. Because a clock can have a period + * which is not a whole number of nanoseconds, it is important + * to use this function when calculating things like timer + * expiry deadlines, rather than attempting to obtain a "period + * in nanoseconds" value and then multiplying that by a number + * of ticks. + * + * The result could in theory be too large to fit in a 64-bit + * value if the number of ticks and the clock period are both + * large; to avoid overflow the result will be saturated to INT64_MAX + * (because this is the largest valid input to the QEMUTimer APIs). + * Since INT64_MAX nanoseconds is almost 300 years, anything with + * an expiry later than that is in the "will never happen" category + * and callers can reasonably not special-case the saturated result. + */ +static inline uint64_t clock_ticks_to_ns(const Clock *clk, uint64_t ticks) +{ + uint64_t ns_low, ns_high; + + /* + * clk->period is the period in units of 2^-32 ns, so + * (clk->period * ticks) is the required length of time in those + * units, and we can convert to nanoseconds by multiplying by + * 2^32, which is the same as shifting the 128-bit multiplication + * result right by 32. + */ + mulu64(&ns_low, &ns_high, clk->period, ticks); + if (ns_high & MAKE_64BIT_MASK(31, 33)) { + return INT64_MAX; + } + return ns_low >> 32 | ns_high << 32; +} + /** * clock_is_enabled: * @clk: a clock --=20 2.26.2