Re: [PATCH 3/3] serial: 8250: omap: restore registers on shutdown

[Peter Hurley <peter@hurleysoftware.com>]

On 08/03/2015 12:54 PM, Sebastian Andrzej Siewior wrote:
> On 08/03/2015 06:34 PM, Peter Hurley wrote:
>> Hi Sebastian,
> 
> Hi Peter,
> 
>>>  struct old_serial_port {
>>> diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
>>> index d9a37191a1ae..12249125a218 100644
>>> --- a/drivers/tty/serial/8250/8250_omap.c
>>> +++ b/drivers/tty/serial/8250/8250_omap.c
>>> @@ -100,9 +100,9 @@ struct omap8250_priv {
>>>  	u8 wer;
>>>  	u8 xon;
>>>  	u8 xoff;
>>> -	u8 delayed_restore;
>>>  	u16 quot;
>>>  
>>> +	wait_queue_head_t termios_wait;
>>>  	bool is_suspending;
>>>  	int wakeirq;
>>>  	int wakeups_enabled;
>>> @@ -256,18 +256,6 @@ static void omap8250_update_mdr1(struct uart_8250_port *up,
>>>  static void omap8250_restore_regs(struct uart_8250_port *up)
>>>  {
>>>  	struct omap8250_priv *priv = up->port.private_data;
>>> -	struct uart_8250_dma	*dma = up->dma;
>>> -
>>> -	if (dma && dma->tx_running) {
>>> -		/*
>>> -		 * TCSANOW requests the change to occur immediately however if
>>> -		 * we have a TX-DMA operation in progress then it has been
>>> -		 * observed that it might stall and never complete. Therefore we
>>> -		 * delay DMA completes to prevent this hang from happen.
>>> -		 */
>>> -		priv->delayed_restore = 1;
>>> -		return;
>>> -	}
>>>  
>>>  	serial_out(up, UART_LCR, UART_LCR_CONF_MODE_B);
>>>  	serial_out(up, UART_EFR, UART_EFR_ECB);
>>> @@ -309,6 +297,7 @@ static void omap8250_restore_regs(struct uart_8250_port *up)
>>>  	up->port.ops->set_mctrl(&up->port, up->port.mctrl);
>>>  }
>>>  
>>> +static void omap_8250_dma_tx_complete(void *param);
>>>  /*
>>>   * OMAP can use "CLK / (16 or 13) / div" for baud rate. And then we have have
>>>   * some differences in how we want to handle flow control.
>>> @@ -322,6 +311,7 @@ static void omap_8250_set_termios(struct uart_port *port,
>>>  	struct omap8250_priv *priv = up->port.private_data;
>>>  	unsigned char cval = 0;
>>>  	unsigned int baud;
>>> +	unsigned int complete_dma = 0;
>>>  
>>>  	switch (termios->c_cflag & CSIZE) {
>>>  	case CS5:
>>> @@ -473,6 +463,25 @@ static void omap_8250_set_termios(struct uart_port *port,
>>>  		if (termios->c_iflag & IXANY)
>>>  			up->mcr |= UART_MCR_XONANY;
>>>  	}
>>> +
>>> +	if (up->dma && up->dma->tx_running) {
>>> +		struct uart_8250_dma	*dma = up->dma;
>>> +
>>> +		/*
>>> +		 * TCSANOW requests the change to occur immediately however if
>>> +		 * we have a TX-DMA operation in progress then it has been
>>> +		 * observed that it might stall and never complete. Therefore we
>>> +		 * wait until DMA completes to prevent this hang from happen.
>>> +		 */
>>> +
>>> +		dma->tx_running = 2;
>>> +
>>> +		spin_unlock_irq(&up->port.lock);
>>> +		wait_event(priv->termios_wait,
>>> +			   dma->tx_running == 3);
>>
>> Doesn't the dmaengine api offer a race-free way to wait for pending tx dma
>> to complete?
> 
> Not that I know of. You still need to ensure that once that DMA
> completed, nobody triggers another TX transfer before you do what you
> planned. This is ensures by the tx_running != 0 and the spin lock.
> 
>> Maybe we could wrap that in the 8250 dma api?
> 
> You mean a function in 8250-dma API which does what I did just here
> with the wait_event() and the wake_up in the callback? That way I could
> move the termios_wait into the dma struct instead of keeping in the
> omap specific part. I am also not sure if OMAP is the only one that may
> hang here or the other people just didn't notice it yet.

Exactly; and we need to fix DMA wrt x_char anyway.

Going back to the dmaengine api, I think something like this might work
(as a first approximation):

	dma_sync_wait(dma->txchan, dma->tx_cookie);
	dmaengine_pause(dma->txchan);

	/* remainder of set_termios */

	dmaengine_resume(dma->txchan);

We could require 8250 core dma to support pause/resume.


>>> +		spin_lock_irq(&up->port.lock);
>>> +		complete_dma = 1;
>>> +	}
>>>  	omap8250_restore_regs(up);
>>>  
>>>  	spin_unlock_irq(&up->port.lock);
>>> @@ -488,6 +497,8 @@ static void omap_8250_set_termios(struct uart_port *port,
>>>  	/* Don't rewrite B0 */
>>>  	if (tty_termios_baud_rate(termios))
>>>  		tty_termios_encode_baud_rate(termios, baud, baud);
>>> +	if (complete_dma)
>>> +		omap_8250_dma_tx_complete(up);
>>>  }
>>>  
>>>  /* same as 8250 except that we may have extra flow bits set in EFR */
>>> @@ -869,17 +880,18 @@ static void omap_8250_dma_tx_complete(void *param)
>>>  
>>>  	spin_lock_irqsave(&p->port.lock, flags);
>>>  
>>> +	if (dma->tx_running == 2) {
>>> +		dma->tx_running = 3;
>>> +		wake_up(&priv->termios_wait);
>>> +		goto out;
>>> +	}
>>> +
>>>  	dma->tx_running = 0;
>>>  
>>>  	xmit->tail += dma->tx_size;
>>>  	xmit->tail &= UART_XMIT_SIZE - 1;
>>>  	p->port.icount.tx += dma->tx_size;
>>>  
>>> -	if (priv->delayed_restore) {
>>> -		priv->delayed_restore = 0;
>>> -		omap8250_restore_regs(p);
>>> -	}
>>> -
>>>  	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
>>>  		uart_write_wakeup(&p->port);
>>>  
>>> @@ -899,7 +911,7 @@ static void omap_8250_dma_tx_complete(void *param)
>>>  		p->ier |= UART_IER_THRI;
>>>  		serial_port_out(&p->port, UART_IER, p->ier);
>>>  	}
>>> -
>>> +out:
>>>  	spin_unlock_irqrestore(&p->port.lock, flags);
>>>  }
>>>  
>>> @@ -1216,6 +1228,7 @@ static int omap8250_probe(struct platform_device *pdev)
>>>  			priv->omap8250_dma.rx_size = RX_TRIGGER;
>>>  			priv->omap8250_dma.rxconf.src_maxburst = RX_TRIGGER;
>>>  			priv->omap8250_dma.txconf.dst_maxburst = TX_TRIGGER;
>>> +			init_waitqueue_head(&priv->termios_wait);
>>>  
>>>  			if (of_machine_is_compatible("ti,am33xx"))
>>>  				priv->habit |= OMAP_DMA_TX_KICK;
>>>
> 
> Sebastian
>