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Date: Mon, 03 Aug 2015 18:54:56 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Peter Hurley <peter@...leysoftware.com>
CC: John Ogness <john.ogness@...utronix.de>,
linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Tony Lindgren <tony@...mide.com>, linux-omap@...r.kernel.org,
nsekhar@...com, nm@...com, linux-serial@...r.kernel.org,
Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH 3/3] serial: 8250: omap: restore registers on shutdown
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.
> Regards,
> Peter Hurley
>
>> + 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
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