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

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Date:	Thu, 25 Sep 2014 07:31:32 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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

On 09/25/2014 06:42 AM, Sebastian Andrzej Siewior wrote:
> * Sebastian Andrzej Siewior | 2014-09-24 09:53:46 [+0200]:
> 
>> * Peter Hurley | 2014-09-23 13:03:51 [-0400]:
>>
>>> 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.
> 
> I added 
> 
> |#define BOTH_EMPTY      (UART_LSR_TEMT | UART_LSR_THRE)
> |    trace_printk("delay <%d>\n", (lsr & BOTH_EMPTY) == BOTH_EMPTY ? 1 : 0);
> 
> in my set_termios() and the trace shows
> |              vi-949   [000] d...    70.477002: omap8250_restore_regs: delay <0>
> 
> so no, it does not wait until TX FIFO is empty. It looks like it uses
> TCSANOW instead of TCSADRAIN. And since this looks "legal" I will delay
> it until TX-DMA is complete because it is known to stall the DMA operation.

I just verified that GNU readline6 uses ioctl(TCSETSW, ...) to do the set_termios
(which is the ioctl that libc should use for tcsetattr(TCSADRAIN)).

Maybe this userspace is using a readline()-alike that has a bug by not using the
correct tcsetattr() action?
Or maybe the glibc-equivalent has the bug, and tcsetattr(TCSADRAIN) is not using
ioctl(TCSETSW)?

Regards,
Peter Hurley
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