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[81.2.115.148]) by smtp.gmail.com with ESMTPSA id y11sm34752472wrh.16.2021.02.09.05.20.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Feb 2021 05:20:46 -0800 (PST) From: Peter Maydell To: qemu-arm@nongnu.org, qemu-devel@nongnu.org Subject: [PATCH v2 3/4] clock: Add clock_ns_to_ticks() function Date: Tue, 9 Feb 2021 13:20:39 +0000 Message-Id: <20210209132040.5091-4-peter.maydell@linaro.org> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20210209132040.5091-1-peter.maydell@linaro.org> References: <20210209132040.5091-1-peter.maydell@linaro.org> MIME-Version: 1.0 Received-SPF: pass client-ip=2a00:1450:4864:20::42e; envelope-from=peter.maydell@linaro.org; helo=mail-wr1-x42e.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_PASS=-0.001, T_SPF_HELO_TEMPERROR=0.01 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Edgar E. Iglesias" , Tyrone Ting , Luc Michel , =?utf-8?q?Ph?= =?utf-8?q?ilippe_Mathieu-Daud=C3=A9?= , Havard Skinnemoen Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: "Qemu-devel" Add a clock_ns_to_ticks() function which does the opposite of clock_ticks_to_ns(): given a duration in nanoseconds, it returns the number of clock ticks that would happen in that time. This is useful for devices that have a free running counter register whose value can be calculated when it is read. Signed-off-by: Peter Maydell Reviewed-by: Hao Wu Tested-by: Philippe Mathieu-Daudé Reviewed-by: Luc Michel --- I have made the overflow behaviour here be "wrap", with justification as per the comment; but I'm not 100% set on this. --- docs/devel/clocks.rst | 12 ++++++++++++ include/hw/clock.h | 41 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 53 insertions(+) diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst index f0391e76b4f..956bd147ea0 100644 --- a/docs/devel/clocks.rst +++ b/docs/devel/clocks.rst @@ -360,6 +360,18 @@ 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.) +Obtaining tick counts +--------------------- + +For calculations where you need to know the number of ticks in +a given duration, use ``clock_ns_to_ticks()``. This function handles +possible non-whole-number-of-nanoseconds periods and avoids +potential rounding errors. It will return '0' if the clock is stopped +(i.e. it has period zero). If the inputs imply a tick count that +overflows a 64-bit value (a very long duration for a clock with a +very short period) the output value is truncated, so effectively +the 64-bit output wraps around. + Changing a clock period ----------------------- diff --git a/include/hw/clock.h b/include/hw/clock.h index d7a6673c29e..79c3b7ebe40 100644 --- a/include/hw/clock.h +++ b/include/hw/clock.h @@ -286,6 +286,47 @@ static inline uint64_t clock_ticks_to_ns(const Clock *clk, uint64_t ticks) return ns_low >> 32 | ns_high << 32; } +/** + * clock_ns_to_ticks: + * @clk: the clock to query + * @ns: duration in nanoseconds + * + * Returns the number of ticks this clock would make in the given + * number of nanoseconds. Because a clock can have a period which + * is not a whole number of nanoseconds, it is important to use this + * function rather than attempting to obtain a "period in nanoseconds" + * value and then dividing the duration by that value. + * + * If the clock is stopped (ie it has period zero), returns 0. + * + * For some inputs the result could overflow a 64-bit value (because + * the clock's period is short and the duration is long). In these + * cases we truncate the result to a 64-bit value. This is on the + * assumption that generally the result is going to be used to report + * a 32-bit or 64-bit guest register value, so wrapping either cannot + * happen or is the desired behaviour. + */ +static inline uint64_t clock_ns_to_ticks(const Clock *clk, uint64_t ns) +{ + /* + * ticks = duration_in_ns / period_in_ns + * = ns / (period / 2^32) + * = (ns * 2^32) / period + * The hi, lo inputs to divu128() are (ns << 32) as a 128 bit value. + */ + uint64_t lo = ns << 32; + uint64_t hi = ns >> 32; + if (clk->period == 0) { + return 0; + } + /* + * Ignore divu128() return value as we've caught div-by-zero and don't + * need different behaviour for overflow. + */ + divu128(&lo, &hi, clk->period); + return lo; +} + /** * clock_is_enabled: * @clk: a clock