From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754030AbdBFO66 (ORCPT ); Mon, 6 Feb 2017 09:58:58 -0500 Received: from mail-wm0-f65.google.com ([74.125.82.65]:33377 "EHLO mail-wm0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751788AbdBFO64 (ORCPT ); Mon, 6 Feb 2017 09:58:56 -0500 Subject: Re: Fractional divider on the Atmel USART controller To: Romain Izard , linux-arm-kernel , linux-serial@vger.kernel.org, LKML , Ludovic Desroches , Nicolas Ferre References: <20170206134256.tp5rgx6axd532gp3@rfolt0960.corp.atmel.com> From: Richard Genoud Message-ID: Date: Mon, 6 Feb 2017 15:58:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.6.0 MIME-Version: 1.0 In-Reply-To: <20170206134256.tp5rgx6axd532gp3@rfolt0960.corp.atmel.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Ludovic, On 06/02/2017 14:42, Ludovic Desroches wrote: > Hello Romain, > > On Mon, Feb 06, 2017 at 12:56:42PM +0100, Romain Izard wrote: >> Hello, >> >> On Atmel SAMA5D2, when trying to configure a serial port for 3 Mbauds >> operation, I do not always get the requested baud rate. If the hardware >> flow control is disabled by software, the line works correctly. But if I >> set the crtscts option, the line does not work, and after checking the >> line I can observe that the signal is sent at 2.6 Mbauds. >> >> This is due to the code used to manage fractional baud rate divisor: the >> existing code prevents the fractional bits from being used if the line >> is not configured in normal mode. This case occurs when the hardware >> flow control or the RS485 mode is set. >> >> If I apply the following patch to drivers/tty/serial/atmel_serial.c, >> I get the required baudrate. >> >> 8<---------------------------------------------------------------- >> >> @@ -2204,14 +2204,13 >> * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8)) >> * Currently, OVER is always set to 0 so we get >> * baudrate = selected clock / (16 * (CD + FP / 8)) >> * then >> * 8 CD + FP = selected clock / (2 * baudrate) >> */ >> - if (atmel_port->has_frac_baudrate && >> - (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) { >> + if (atmel_port->has_frac_baudrate) { >> div = DIV_ROUND_CLOSEST(port->uartclk, baud * 2); >> cd = div >> 3; >> fp = div & ATMEL_US_FP_MASK; >> } else { >> cd = uart_get_divisor(port, baud); >> } >> >> 8<---------------------------------------------------------------- >> >> Unfortunately, I know that this will work on SAMA5D2, but this driver is >> used for many other Atmel chips. I do not know if the existing code is >> meant to respect a known limitation on other devices that use the same >> controller, or if it is just a bug. > > It depends on the device used. In the SAMA5D4 datasheet, it is > mentionned: "This feature is only available when using USART normal mode". It seems you have an old datasheet version, because in table 60.2 (page 1784) of SAMA5D4 datasheet (rev D), there's : Section 43.6.1.2 “Fractional Baud Rate in Asynchronous Mode”: added warning “When the value of field FP is greater than 0...”; removed sentence “This feature is only available when using USART normal mode.” So I guess this should also work on SAMA5D4 ? > In the SAMA5D2 datasheet, this constraint is no longer mentionned you > confirm that is works, that is great. > >> >> Ludovic, Nicolas, what is your opinion on that matter? Should I just >> propose this as a patch, or is it necessary to add a limitation for >> supported devices only ? > > Yes, only SAMA5D2 supports fractionnal baudrate in other modes than the > normal one. A new compatible string may be added, I am not sure you have > enough information in atmel_get_ip_name... I checked the version numbers on some atmel platforms I have: AT91SAM9G35 / SAMA5D3: 0x502 AT91SAM9G20: 0x10330 SAMA5D2: 0x814 Ludovic, could you please check the version number of the SAMA5D4 USART (if you have one lying around) ?