From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752025AbbKIWFT (ORCPT ); Mon, 9 Nov 2015 17:05:19 -0500 Received: from mail-io0-f172.google.com ([209.85.223.172]:36242 "EHLO mail-io0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751601AbbKIWFO (ORCPT ); Mon, 9 Nov 2015 17:05:14 -0500 Subject: Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250 To: "Matwey V. Kornilov" 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> Cc: Greg KH , jslaby@suse.com, linux-serial@vger.kernel.org, linux-kernel From: Peter Hurley Message-ID: <56411896.6060801@hurleysoftware.com> Date: Mon, 9 Nov 2015 17:05:10 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). > 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