linux-kernel - Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx

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Message-ID: <20140924075346.GB19565@linutronix.de>
Date:	Wed, 24 Sep 2014 09:53:46 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Frans Klaver <frans.klaver@...ns.com>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	tony@...mide.com, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, balbi@...com,
	linux-serial@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround
 for AM33xx

* Peter Hurley | 2014-09-23 13:03:51 [-0400]:

>readline() does this; it 'saves' the caller's termios, sets termios
>for non-canonical reads, reads one char, and 'restores' the caller's
>termios.

interresting, thanks. I guess I would need to opimize this a little so
the baudrate isn't going to 0 and back to the requested baudrate.

>The tty core calls the driver's wait_until_sent() method before changing
>the termios (if TCSADRAIN is used for tcsetattr(), which I think for readline()
>it does).

The interresting difference is that when I take the yocto RFS and chroot
into from Debian then I don't this problem. Not sure if this is really
readline or something else…

>But DMA is cheating if the UART driver's tx_empty() method is saying the
>transmitter is empty while TX DMA is still running.
This shouldn't be the case. But I will check this once I able to.
After TX-DMA is done, "xmit->tail" is updated and port.icount.tx is
incremented. At this time the TX FIFO is still full (up to 64 bytes) and
I set UART_IER_THRI to wait until TX FIFO is empty so I can disable
runtime-pm. Therefore I would assume LSR does not say BOTH_EMPTY until
the FIFO is empty.

>Regards,
>Peter Hurley

Sebastian
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