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

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Date:	Thu, 11 Sep 2014 08:32:58 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	tony@...mide.com, balbi@...com, gregkh@...uxfoundation.org,
	Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround
 for AM33xx

On 09/11/2014 07:42 AM, Sebastian Andrzej Siewior wrote:
> I also need a watchdog timer for TX since it seems that on omap3 the
> DMA engine suddenly forgets to continue with DMA…

One difference I noticed between the omap driver and the 8250 driver is
the way modem status interrupts are handled.

The omap driver only checks modem status for the UART_IIR_MSI interrupt type.
The 8250 driver checks modem status at every interrupt (other than NO_INT).

I think the UART_MSR_DCTS bit always reflects that the CTS input has changed
between reads of the MSR _even if auto CTS is on_. So perhaps the hardware
is being stopped by uart_handle_cts_change() when auto CTS is on?

Regards,
Peter Hurley

[The UPF_HARD_FLOW thing was pretty much just done for omap even though
8250 already had auto CTS/auto RTS. Serial core hardware flow control support
needs a redo as drivers have pretty much tacked stuff on randomly.]
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