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Thu, 5 Jul 2018 06:56:48 +0000 Date: Thu, 5 Jul 2018 14:54:58 +0800 From: Jisheng Zhang To: Andy Shevchenko Cc: Greg Kroah-Hartman , Jiri Slaby , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Subject: Re: [PATCH v2 3/3] serial: 8250_dw: add fractional divisor support Message-ID: <20180705145458.7e7d9b9f@xhacker.debian> In-Reply-To: <20180705143921.6a8aeb50@xhacker.debian> References: <20180704165908.4bb8b090@xhacker.debian> <20180704170310.56772d77@xhacker.debian> <20180705143921.6a8aeb50@xhacker.debian> X-Mailer: Claws Mail 3.16.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Originating-IP: [124.74.246.114] X-ClientProxiedBy: TY2PR02CA0019.apcprd02.prod.outlook.com (2603:1096:404:56::31) To BN6PR03MB2626.namprd03.prod.outlook.com (2603:10b6:404:59::9) X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 5697dae6-d96b-401b-cfa1-08d5e2447b6c X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:(7020095)(4652040)(8989117)(5600053)(711020)(4534165)(4627221)(201703031133081)(201702281549075)(8990107)(2017052603328)(7153060)(7193020);SRVR:BN6PR03MB2626; 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> > > + serial_dl_write(up, quot); > > > > At some point it would be a helper, I think. We can call > > serial8250_do_set_divisor() here. So, perhaps we might export it. > > serial8250_do_set_divisor will drop the frac, that's not we want ;) > And most importantly, serial8250_do_set_divisor() will set a wrong BRD(I) for fractional capable DW uarts. For example, clk = 25MHZ, baud = 115200. In fractional capable DW uarts, we should set BRD(I) as 25000000/(16*115200) = 13 but serial8250_do_set_divisor() will set BRD(I) as DIV_ROUND_CLOSEST(25*1000000, 16*115200)) = 14 Thanks