linux-kernel - Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250

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Date:	Mon, 9 Nov 2015 16:30:48 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Matwey V. Kornilov" <matwey@....msu.ru>
Cc:	Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
	linux-serial@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250

On 11/09/2015 10:45 AM, Matwey V. Kornilov wrote:
> 2015-11-09 17:40 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
>> On 11/08/2015 05:52 AM, Matwey V. Kornilov wrote:
>>> 2015-11-07 19:03 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
>>>> 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
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