From patchwork Sat Feb 19 15:28:17 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: =?utf-8?q?Marek_Beh=C3=BAn?= X-Patchwork-Id: 544377 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CDE91C433FE for ; Sat, 19 Feb 2022 15:28:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242574AbiBSP3L (ORCPT ); Sat, 19 Feb 2022 10:29:11 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:43978 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242556AbiBSP3J (ORCPT ); Sat, 19 Feb 2022 10:29:09 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 244575D641; Sat, 19 Feb 2022 07:28:38 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id C4BE8B80968; Sat, 19 Feb 2022 15:28:36 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8C8C7C340EF; Sat, 19 Feb 2022 15:28:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1645284515; bh=r3iqPCoANxpwhUGnTd+JNF0yIf5d3A5itF0q+px7HIs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=cAS4vy5466az6eQ1Pqk0VeqKGk92+l2nyMfxbqgJjHRIoJScndcXCnCIySMhua7e+ nBcbZDPwTh5C2ELBQod8QIoa77a5qRx53K+6GV5Hw14hgDbJuYAqjCzELSqt/C0Vr+ ezTay4Z2icQUb+e8FfgPJPSZ1SJBQ5Pzq/haeOg4W4baApysYinOvgME0KLTLMAvl2 /d5UEGMqpm4IsqjOppJXhq6BEJ1gHV1HdZZ+wi9/E+2a/Ktuqbl5i2gbIl19KIxhAe RHmMzHCYWIvXpIqxOf/J6VjUNWYOgFgHm4mMshjTJGhN0U3LHzaOGbM/7J8LXR8wS2 NYnELHWMaHVmA== From: =?utf-8?q?Marek_Beh=C3=BAn?= To: Greg Kroah-Hartman , Stephen Boyd Cc: Gregory Clement , =?utf-8?q?Pali_Roh=C3=A1r?= , linux-clk@vger.kernel.org, linux-serial@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, =?utf-8?q?Marek_Beh=C3=BAn?= Subject: [PATCH v9 5/6] serial: mvebu-uart: implement support for baudrates higher than 230400 Bd Date: Sat, 19 Feb 2022 16:28:17 +0100 Message-Id: <20220219152818.4319-6-kabel@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20220219152818.4319-1-kabel@kernel.org> References: <20220219152818.4319-1-kabel@kernel.org> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org From: Pali Rohár Implement simple usage of fractional divisor. When main divisor D is too large to represent requested baudrate then use divisor M from the fractional divisor feature. All the M prescalers are set to the same and maximal value 63, so the fractional part of the fractional divisor is not used at all. We also determine upper limit for possible baudrates. Experiments show that UART at baudrate 1500000 Bd with this configuration is stable. So there is no need to implement complicated calculation of fractional coefficients yet. To use this feature with higher baudrates, it is required to use UART clock provided by UART clock driver. Default boot xtal clock is not capable of higher baudrates. Signed-off-by: Pali Rohár Reviewed-by: Marek Behún Signed-off-by: Marek Behún --- drivers/tty/serial/mvebu-uart.c | 83 ++++++++++++++++++++++++++------- 1 file changed, 65 insertions(+), 18 deletions(-) diff --git a/drivers/tty/serial/mvebu-uart.c b/drivers/tty/serial/mvebu-uart.c index 56278b29f5f5..2e9263888ddc 100644 --- a/drivers/tty/serial/mvebu-uart.c +++ b/drivers/tty/serial/mvebu-uart.c @@ -99,6 +99,7 @@ #define UART_OSAMP 0x14 #define OSAMP_DEFAULT_DIVISOR 16 #define OSAMP_DIVISORS_MASK 0x3F3F3F3F +#define OSAMP_MAX_DIVISOR 63 #define MVEBU_NR_UARTS 2 @@ -479,18 +480,60 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud) return -EOPNOTSUPP; /* - * The baudrate is derived from the UART clock thanks to two divisors: - * > D ("baud generator"): can divide the clock from 2 to 2^10 - 1. - * > M ("fractional divisor"): allows a better accuracy for - * baudrates higher than 230400. + * The baudrate is derived from the UART clock thanks to divisors: + * > d1 * d2 ("TBG divisors"): can divide only TBG clock from 1 to 6 + * > D ("baud generator"): can divide the clock from 1 to 1023 + * > M ("fractional divisor"): allows a better accuracy (from 1 to 63) * - * As the derivation of M is rather complicated, the code sticks to its - * default value (x16) when all the prescalers are zeroed, and only - * makes use of D to configure the desired baudrate. + * Exact formulas for calculating baudrate: + * + * with default x16 scheme: + * baudrate = xtal / (d * 16) + * baudrate = tbg / (d1 * d2 * d * 16) + * + * with fractional divisor: + * baudrate = 10 * xtal / (d * (3 * (m1 + m2) + 2 * (m3 + m4))) + * baudrate = 10 * tbg / (d1*d2 * d * (3 * (m1 + m2) + 2 * (m3 + m4))) + * + * Oversampling value: + * osamp = (m1 << 0) | (m2 << 8) | (m3 << 16) | (m4 << 24); + * + * Where m1 controls number of clock cycles per bit for bits 1,2,3; + * m2 for bits 4,5,6; m3 for bits 7,8 and m4 for bits 9,10. + * + * To simplify baudrate setup set all the M prescalers to the same + * value. For baudrates 9600 Bd and higher, it is enough to use the + * default (x16) divisor or fractional divisor with M = 63, so there + * is no need to use real fractional support (where the M prescalers + * are not equal). + * + * When all the M prescalers are zeroed then default (x16) divisor is + * used. Default x16 scheme is more stable than M (fractional divisor), + * so use M only when D divisor is not enough to derive baudrate. + * + * Member port->uartclk is either xtal clock rate or TBG clock rate + * divided by (d1 * d2). So d1 and d2 are already set by the UART clock + * driver (and UART driver itself cannot change them). Moreover they are + * shared between both UARTs. */ + m_divisor = OSAMP_DEFAULT_DIVISOR; d_divisor = DIV_ROUND_CLOSEST(port->uartclk, baud * m_divisor); + if (d_divisor > BRDV_BAUD_MAX) { + /* + * Experiments show that small M divisors are unstable. + * Use maximal possible M = 63 and calculate D divisor. + */ + m_divisor = OSAMP_MAX_DIVISOR; + d_divisor = DIV_ROUND_CLOSEST(port->uartclk, baud * m_divisor); + } + + if (d_divisor < 1) + d_divisor = 1; + else if (d_divisor > BRDV_BAUD_MAX) + d_divisor = BRDV_BAUD_MAX; + spin_lock_irqsave(&mvebu_uart_lock, flags); brdv = readl(port->membase + UART_BRDV); brdv &= ~BRDV_BAUD_MASK; @@ -500,6 +543,9 @@ static int mvebu_uart_baud_rate_set(struct uart_port *port, unsigned int baud) osamp = readl(port->membase + UART_OSAMP); osamp &= ~OSAMP_DIVISORS_MASK; + if (m_divisor != OSAMP_DEFAULT_DIVISOR) + osamp |= (m_divisor << 0) | (m_divisor << 8) | + (m_divisor << 16) | (m_divisor << 24); writel(osamp, port->membase + UART_OSAMP); return 0; @@ -529,14 +575,16 @@ static void mvebu_uart_set_termios(struct uart_port *port, port->ignore_status_mask |= STAT_RX_RDY(port) | STAT_BRK_ERR; /* - * Maximal divisor is 1023 * 16 when using default (x16) scheme. - * Maximum achievable frequency with simple baudrate divisor is 230400. - * Since the error per bit frame would be of more than 15%, achieving - * higher frequencies would require to implement the fractional divisor - * feature. + * Maximal divisor is 1023 and maximal fractional divisor is 63. And + * experiments show that baudrates above 1/80 of parent clock rate are + * not stable. So disallow baudrates above 1/80 of the parent clock + * rate. If port->uartclk is not available, then + * mvebu_uart_baud_rate_set() fails, so values min_baud and max_baud + * in this case do not matter. */ - min_baud = DIV_ROUND_UP(port->uartclk, 1023 * 16); - max_baud = 230400; + min_baud = DIV_ROUND_UP(port->uartclk, BRDV_BAUD_MAX * + OSAMP_MAX_DIVISOR); + max_baud = port->uartclk / 80; baud = uart_get_baud_rate(port, termios, old, min_baud, max_baud); if (mvebu_uart_baud_rate_set(port, baud)) { @@ -1395,14 +1443,14 @@ static int mvebu_uart_clock_probe(struct platform_device *pdev) * Calculate the smallest TBG d1 and d2 divisors that * still can provide 9600 baudrate. */ - d1 = DIV_ROUND_UP(rate, 9600 * OSAMP_DEFAULT_DIVISOR * + d1 = DIV_ROUND_UP(rate, 9600 * OSAMP_MAX_DIVISOR * BRDV_BAUD_MAX); if (d1 < 1) d1 = 1; else if (d1 > CLK_TBG_DIV1_MAX) d1 = CLK_TBG_DIV1_MAX; - d2 = DIV_ROUND_UP(rate, 9600 * OSAMP_DEFAULT_DIVISOR * + d2 = DIV_ROUND_UP(rate, 9600 * OSAMP_MAX_DIVISOR * BRDV_BAUD_MAX * d1); if (d2 < 1) d2 = 1; @@ -1417,8 +1465,7 @@ static int mvebu_uart_clock_probe(struct platform_device *pdev) } /* Skip clock source which cannot provide 9600 baudrate */ - if (rate > 9600 * OSAMP_DEFAULT_DIVISOR * BRDV_BAUD_MAX * d1 * - d2) + if (rate > 9600 * OSAMP_MAX_DIVISOR * BRDV_BAUD_MAX * d1 * d2) continue; /*