From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752176AbbKJLgW (ORCPT ); Tue, 10 Nov 2015 06:36:22 -0500 Received: from mail-wm0-f53.google.com ([74.125.82.53]:33451 "EHLO mail-wm0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751572AbbKJLgU (ORCPT ); Tue, 10 Nov 2015 06:36:20 -0500 MIME-Version: 1.0 In-Reply-To: <56411896.6060801@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> <56411896.6060801@hurleysoftware.com> From: "Matwey V. Kornilov" Date: Tue, 10 Nov 2015 14:35:59 +0300 X-Google-Sender-Auth: jyGIQ5oGPw4bzG2MOFxFqY56Sfc 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 1:05 GMT+03:00 Peter Hurley : > On 11/09/2015 04:43 PM, Matwey V. Kornilov wrote: >> 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. > > No; start_tx() must cause tx to become started, but tx does not > have to have _already_ started when start_tx() returns. > > It would be very inefficient for start_tx() to _guarantee_ tx has > already started _before_ returning. Note the 8250 driver merely > writes to IER (which could be buffered and bridged). > Thank you, now this is becoming clear to me. >> And that could be not true, if the timer is used. > > It's true that using a timer will be more complex with more state > to manage, but being unable to service interrupts with this cpu for > milliseconds is unacceptable. > > Regards, > Peter Hurley > -- With best regards, Matwey V. Kornilov. Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia 119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382