From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752512AbdLFGFt (ORCPT ); Wed, 6 Dec 2017 01:05:49 -0500 Received: from relmlor2.renesas.com ([210.160.252.172]:17583 "EHLO relmlie1.idc.renesas.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752305AbdLFGFp (ORCPT ); Wed, 6 Dec 2017 01:05:45 -0500 X-IronPort-AV: E=Sophos;i="5.45,367,1508770800"; d="scan'208";a="264300141" Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=kuninori.morimoto.gx@renesas.com; Message-ID: <87374oz9f9.wl%kuninori.morimoto.gx@renesas.com> From: Kuninori Morimoto Subject: [PATCH v2] drm: rcar-du: calculate DPLLCR to be more small jitter User-Agent: Wanderlust/2.15.9 Emacs/24.3 Mule/6.0 To: Laurent Pinchart , David Airlie Cc: dri-devel@lists.freedesktop.org, linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; 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In academic generalism, VCO should be maximum to be more small jitter. In high frequency clock, jitter will be large impact. Thus, selecting Hi VCO is general theory. fin fvco fout fclkout in --> [1/M] --> |PD| -> [LPF] -> [VCO] -> [1/P] -+-> [1/FDPLL] -> out +-> | | | | | +-----------------[1/N]<-------------+ fclkout = fvco / P / FDPLL -- (1) In PD, it will loop until fin/M = fvco/P/N fvco = fin * P * N / M -- (2) (1) + (2) indicates, fclkout = fin * N / M / FDPLL In this device, N = (n + 1), M = (m + 1), P = 2, thus fclkout = fin * (n + 1) / (m + 1) / FDPLL This is the datasheet formula. One note here is that it should be 2000 < fvco < 4096MHz To be smaller jitter, fvco should be maximum, in other words, N as large as possible, M as small as possible driver should select. Here, basically M=1. This patch do it. Reported-by: HIROSHI INOSE Signed-off-by: Kuninori Morimoto --- v1 -> v2 - tidyup typo on git-log "fout" -> "fclkout" - tidyup for loop terminate condition 40 -> 38 for n drivers/gpu/drm/rcar-du/rcar_du_crtc.c | 36 ++++++++++++++++++++++++++++++++-- 1 file changed, 34 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c index b492063..57479c9 100644 --- a/drivers/gpu/drm/rcar-du/rcar_du_crtc.c +++ b/drivers/gpu/drm/rcar-du/rcar_du_crtc.c @@ -125,8 +125,40 @@ static void rcar_du_dpll_divider(struct rcar_du_crtc *rcrtc, unsigned int m; unsigned int n; - for (n = 39; n < 120; n++) { - for (m = 0; m < 4; m++) { + /* + * fin fvco fout fclkout + * in --> [1/M] --> |PD| -> [LPF] -> [VCO] -> [1/P] -+-> [1/FDPLL] -> out + * +-> | | | + * | | + * +-----------------[1/N]<-------------+ + * + * fclkout = fvco / P / FDPLL -- (1) + * + * fin/M = fvco/P/N + * + * fvco = fin * P * N / M -- (2) + * + * (1) + (2) indicates + * + * fclkout = fin * N / M / FDPLL + * + * NOTES + * N = (n + 1), M = (m + 1), P = 2 + * 2000 < fvco < 4096Mhz + * Basically M=1 + * + * To be small jitter, + * N : as large as possible + * M : as small as possible + */ + for (m = 0; m < 4; m++) { + for (n = 119; n > 38; n--) { + unsigned long long fvco = input * 2 * (n + 1) / (m + 1); + + if ((fvco < 2000) || + (fvco > 4096000000ll)) + continue; + for (fdpll = 1; fdpll < 32; fdpll++) { unsigned long output; -- 1.9.1