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

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Date:	Mon, 9 Nov 2015 17:05:10 -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 04:43 PM, Matwey V. Kornilov wrote:
> 2015-11-10 0:30 GMT+03:00 Peter Hurley <peter@...leysoftware.com>:
>> 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
>>
> 
> 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
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