Messages in this thread | | | From | "Matwey V. Kornilov" <> | Date | Tue, 10 Nov 2015 00:43:54 +0300 | Subject | Re: [PATCH v2 3/3] tty: Add software emulated RS485 support for 8250 |
| |
2015-11-10 0:30 GMT+03:00 Peter Hurley <peter@hurleysoftware.com>: > On 11/09/2015 10:45 AM, Matwey V. Kornilov wrote: >> 2015-11-09 17:40 GMT+03:00 Peter Hurley <peter@hurleysoftware.com>: >>> On 11/08/2015 05:52 AM, Matwey V. Kornilov wrote: >>>> 2015-11-07 19:03 GMT+03:00 Peter Hurley <peter@hurleysoftware.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. 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
| |