From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751520AbbKIVoS (ORCPT ); Mon, 9 Nov 2015 16:44:18 -0500 Received: from mail-wm0-f43.google.com ([74.125.82.43]:36535 "EHLO mail-wm0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750792AbbKIVoQ (ORCPT ); Mon, 9 Nov 2015 16:44:16 -0500 MIME-Version: 1.0 In-Reply-To: <56411088.9050304@hurleysoftware.com> References: <1446890976-1817-1-git-send-email-matwey@sai.msu.ru> <1446890976-1817-3-git-send-email-matwey@sai.msu.ru> <563E20DE.7000209@hurleysoftware.com> <5640B071.8000806@hurleysoftware.com> <56411088.9050304@hurleysoftware.com> From: "Matwey V. Kornilov" Date: Tue, 10 Nov 2015 00:43:54 +0300 X-Google-Sender-Auth: dg-CYo1-v2J_Jz2sT2nEsd5j120 Message-ID: Subject: Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250 To: Peter Hurley Cc: Greg KH , jslaby@suse.com, linux-serial@vger.kernel.org, linux-kernel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 2015-11-10 0:30 GMT+03:00 Peter Hurley : > On 11/09/2015 10:45 AM, Matwey V. Kornilov wrote: >> 2015-11-09 17:40 GMT+03:00 Peter Hurley : >>> On 11/08/2015 05:52 AM, Matwey V. Kornilov wrote: >>>> 2015-11-07 19:03 GMT+03:00 Peter Hurley : >>>>> On 11/07/2015 05:09 AM, Matwey V. Kornilov wrote: > > [...] > >>>>>> +static void serial8250_rs485_start_tx(struct uart_8250_port *p) >>>>>> +{ >>>>>> + if (p->capabilities & UART_CAP_HW485 || !(p->port.rs485.flags & SER_RS485_ENABLED)) >>>>>> + return; >>>>>> + >>>>>> + if (p->port.rs485.flags & SER_RS485_RTS_ON_SEND) { >>>>>> + serial_port_out(&p->port, UART_MCR, UART_MCR_RTS); >>>>>> + if (p->port.rs485.delay_rts_before_send > 0) >>>>>> + mdelay(p->port.rs485.delay_rts_before_send); >>>>> >>>>> So irqs are off for x msecs, and this cpu can't be used for anything else now? >>>>> I think this needs to be solved differently; maybe with a timer? >>>> >>>> Call of serial8250_start_tx is wrapped with spin_lock_irq in serial_core.c:2154 >>> >>> Yep, which is why I pointed out "irqs are off for x msecs". >>> >>>> I've tried to use msleep instead of mdelay but got "BUG: scheduling >>>> while atomic". >>> >>> Right, can't sleep while irqs are off, which is why I suggested something >>> like a timer. >> >> I am not sure that understand you correctly. Do you think that the >> following would be ok? >> >> wait_queue_head_t wait; >> init_waitqueue_head(&wait); >> wait_event_timeout(wait, 0, p->port.rs485.delay_rts_before_send * HZ / 1000); > > Except for spinning, there is no way to wait-in-place with irqs off. > > You'll need to do something more complex, like > 1. raise RTS > 2. start a timer _and return early without starting tx_ > 3. timer goes off, handler actually starts tx > I think this could lead to race conditions. AFAIU when the kernel calls ops->start_tx(uport) and the function returns, then it is supposed that the tx has been started. And that could be not true, if the timer is used. -- With best regards, Matwey V. Kornilov. Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia 119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382