linux-kernel - Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

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Date:	Thu, 17 Jul 2014 18:06:59 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	balbi@...com
CC:	Peter Hurley <peter@...leysoftware.com>,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	mika.westerberg@...ux.intel.com
Subject: Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

On 07/17/2014 06:02 PM, Felipe Balbi wrote:
>> diff --git a/drivers/tty/serial/8250/8250_core.c
>> b/drivers/tty/serial/8250/8250_core.c index 2e4a93b..480a1c0
>> 100644 --- a/drivers/tty/serial/8250/8250_core.c +++
>> b/drivers/tty/serial/8250/8250_core.c @@ -1283,6 +1283,9 @@
>> static inline void __stop_tx(struct uart_8250_port *p) if (p->ier
>> & UART_IER_THRI) { p->ier &= ~UART_IER_THRI; serial_out(p,
>> UART_IER, p->ier); + +		pm_runtime_mark_last_busy(p->port.dev); +
>> pm_runtime_put_autosuspend(p->port.dev); } }
>> 
>> @@ -1310,12 +1313,12 @@ static void serial8250_start_tx(struct
>> uart_port *port) struct uart_8250_port *up = container_of(port,
>> struct uart_8250_port, port);
>> 
>> -	pm_runtime_get_sync(port->dev); if (up->dma &&
>> !serial8250_tx_dma(up)) { goto out; } else if (!(up->ier &
>> UART_IER_THRI)) { up->ier |= UART_IER_THRI; +
>> pm_runtime_get_sync(port->dev); serial_port_out(port, UART_IER,
>> up->ier);
>> 
>> if (up->bugs & UART_BUG_TXEN) { unsigned char lsr;
> 
> this looks better. So we get on start_tx() and put on stop_tx().
> 
>> @@ -1500,9 +1503,10 @@ void serial8250_tx_chars(struct
>> uart_8250_port *up) uart_write_wakeup(port);
>> 
>> DEBUG_INTR("THRE..."); - +#if 0 if (uart_circ_empty(xmit)) 
>> __stop_tx(up); +#endif } EXPORT_SYMBOL_GPL(serial8250_tx_chars);
> 
> is it so that start_tx() gets called one and stop_tx() might be
> called N times ? That looks unbalanced to me. If the calls are
> balanced, then you shouldn't need to care because pm_runtime will
> handle reference counting for you, right?

No, this is okay. If you look, it checks for "up->ier &
UART_IER_THRI". On the second invocation it will see that this bit is
already set and therefore won't call get_sync() for the second time.
That bit is removed in the _stop_tx() path.

>> and now I need to come up with something that is not if (port !=
>> omap) for that #if 0 block. The code disables the TX FIFO empty
>> interrupt once the transfer is complete. I want to call
>> __stop_tx() once the tx fifo is empty. Felipe, Would a check for
>> dev->power.use_autosuspend be the right thing to do?
> 
> probably not, as that's internal to the pm_runtime code. But I
> wonder if start/stop tx calls are balanced, if they are then we're
> good. Unless I'm missing something else.

Do you have other ideas? It doesn't look like this is exported at all.
If we call _stop_tx() right away, then we have 64 bytes in the TX fifo
in the worst case. They should be gone "soon" but the HW-flow control
may delay it (in theory for a long time)).

Sebastian
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