From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mout-p-201.mailbox.org (mout-p-201.mailbox.org [80.241.56.171]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 22F1620EB for ; Sat, 27 May 2023 13:37:10 +0000 (UTC) Received: from smtp2.mailbox.org (smtp2.mailbox.org [IPv6:2001:67c:2050:b231:465::2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-384) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mout-p-201.mailbox.org (Postfix) with ESMTPS id 4QT2fc5X1Cz9sjD; Sat, 27 May 2023 15:28:08 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oltmanns.dev; s=MBO0001; t=1685194088; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=kAyuiYKxcOsDPYn8J7+22xTQEqCLXxVio4CMAPjFVu4=; b=K5iz7QYp5ex0Cwa0rC4vw6obr1zLGTOIBLIkf8BWadDS2CYk6je4It0aKiqgtPz+9ipwmw ioYzBFIwYm8VY260NVUH9czW9S0Ifd/IE7I0TP/Y/bh/EGdj/0jDxHViWOR+zlSEkP7HE2 mdoAwYibq9TUuEyHchPGFFYwbKk135+hf3zhNF81UeYWatiySf6fVCM1WLtj9FW8fmQQ2u OOFE8v6bSXgayd1sZUk62kKMwUi057t5mbUaBuyaH4Y7ap/J64TKc61iyB1Gypb0L+GfiQ ggGWBD9/uVnG2Vpm4svyq+au+C+sw0bJ8b3dXKf0yNQL+zpH/JPVbYh4TVhckA== From: Frank Oltmanns To: linux-arm-kernel@lists.infradead.org, linux-clk@vger.kernel.org, linux-kernel@vger.kernel.org, linux-sunxi@lists.linux.dev Cc: Frank Oltmanns , Andre Przywara , Chen-Yu Tsai , Icenowy Zheng , Jernej Skrabec , Maxime Ripard , Michael Turquette , Rob Herring , Samuel Holland , Stephen Boyd Subject: [RFC PATCH 0/3] clk: sunxi-ng: Optimize rate selection for NKM clocks Date: Sat, 27 May 2023 15:27:44 +0200 Message-Id: <20230527132747.83196-1-frank@oltmanns.dev> Precedence: bulk X-Mailing-List: linux-sunxi@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 4QT2fc5X1Cz9sjD I would like to bring your attention to the current process of setting the rate of an NKM clock. As it stands, when setting the rate of an NKM clock, the rate nearest but less than or equal to the requested rate is found, instead of the nearest rate. Moreover, ccu_nkm_find_best() is called multiple times (footnote [1]) when setting a rate, each time iterating over all combinations of n, k, and m. In response to this, I propose the following refinements to optimize the NKM clock setting: a. when finding the best rate use the nearest rate, even if it is greater than the requested rate (PATCH 1) b. utilize binary search to find the best rate by going through a precalculated, ordered list of all meaningful combinations of n, k, and m (PATCH 2) To illustrate, consider an NKM clock with min_n = 1, max_n = 16, min_k = 2, max_k = 4, min_m = 1, and max_m = 16. With the current approach, we have to go through 1024 combinations, of which only 275 are actually meaningful (the remaining 749 are combinations that result in the same frequency as other combinations). So, when selecting from these sorted 275 combinations we find the closest rate after 9 instead of 1024 steps. As an example, I calculated the table off-line for the pll-mipi clock of sun50i-a64 (PATCH 3). However, I have identified two other potential strategies: 2. calculate before first use and persist for subsequent uses (i.e. never free, see footnote [2]) 3. calculate before first use and free after setting the rate. Each approach carries its own merits. The one I implemented is the most efficient in terms of computation time but consumes the most memory. The second saves compute time after the initial use, while the third minimizes memory usage at the cost of additional computation. The motivation for these proposed changes lies in the current behavior of rate selection for NKM clocks, which doesn't observe the CLK_SET_RATE_PARENT flag. I.e. it does not select a different rate for the parent clock to find the optimal rate. I believe this is likely due to the fact that selecting a new rate is quite compute intense, as it would involve iterating or calculating rates for the parent clock and then testing each rate with different n, k, and m combinations. As an example, if the parent is an NM clock, we would have to work through the combinations of the parent's factors (the parent's n) and divisor (the parent's m). This results in five nested loops to evaluate all possible rates, an effort that escalates if the parent could also influence the grandparent's rate. In my example case (sun50i-a64) the pll-mipi's parent (pll-video0) has 2048 combinations of n and m, of which 1266 are meaningful because the others result in the same frequency for pll-video0. If we can come up with a way to iterate over the possible rates of a parent, this would eventually allow us to make NKM clocks obey the CLK_SET_RATE_PARENT flag. Because it would only require 11,349 (9 * 1,266) steps instead of 2,097,152 (1,024 * 2,048). Things I considered but don't have a clear answer to: - Is there a specific reason, why currently only clock rates less than the requested rate are considered when setting a new rate? - Do you think it is worth the memory and increased complexity to be able to change the parent's clock rate? I look forward to hearing your thoughts on these proposed changes. Thank you for your time and consideration. Footnotes: [1] Multiple times because ccu_nkm_find_best is (indirectly) called from clk.c in - clk_core_req_round_rate_nolock() - clk_calc_new_rates() (which in turn is called three times for reasons that currently elude me) - clk_change_rate [2] Actually, we could free the memory in a new ccu_nkm_terminate() function, which could become part of ccu_nkm_ops. But if my code searching skills don't betray me, there is currently no other clock that implements the terminate function. Frank Oltmanns (3): clk: sunxi-ng: nkm: Minimize difference when finding rate clk: sunxi-ng: Implement precalculated NKM rate selection clk: sunxi-ng: sun50i-a64: Precalculate NKM combinations for pll-mipi drivers/clk/sunxi-ng/ccu-sun50i-a64.c | 281 ++++++++++++++++++++++++++ drivers/clk/sunxi-ng/ccu_mux.c | 2 +- drivers/clk/sunxi-ng/ccu_nkm.c | 97 +++++++-- drivers/clk/sunxi-ng/ccu_nkm.h | 26 +++ 4 files changed, 385 insertions(+), 21 deletions(-) -- 2.40.1