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 13:42:35 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	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 01:17 PM, Heikki Krogerus wrote:
>> diff --git a/drivers/tty/serial/8250/8250.h b/drivers/tty/serial/8250/8250.h
>> index fbed1636e9c4..09489b391568 100644
>> --- a/drivers/tty/serial/8250/8250.h
>> +++ b/drivers/tty/serial/8250/8250.h
>> @@ -82,6 +82,9 @@ struct serial8250_config {
>>  #define UART_BUG_PARITY	(1 << 4)	/* UART mishandles parity if FIFO enabled */
>>  #define UART_BUG_DMA_RX	(1 << 5)	/* UART needs DMA RX req before there is
>>  					   data in FIFO */
>> +#define UART_BUG_DMA_TX	(1 << 6)	/* UART needs one byte in FIFO for
>> +					   kickstart */
> 
> I don't think we should go ahead with this patch. I'm pretty sure
> this is AM335 specific problem, or at least limited to only few
> platforms. And I don't think we should take any more "BUG" flags.
> 
> We should add hooks like tx_dma and rx_dma to struct uart_8250_dma so
> that the probe drivers can replace serial8250_tx_dma and
> seria8250_rx_dma, like I think Alan already suggested.

Okay. Wasn't aware that Alan already suggested that.
I also need a watchdog timer for TX since it seems that on omap3 the
DMA engine suddenly forgets to continue with DMA…

If this is really what we want, I would need to refactor a few things…

> Let's keep serial8250_tx_dma/rx_dma as the default, and not add any
> quirks to them. Only if there is a very common case should it be
> handled in those. The case of RX req needing to be sent before data in
> FIFO maybe one of those, but I'm no sure.

keep in mind that both (RX & TX bugs/hacks) need also a bit of handling
in the 8250-core so it works together (like the tx_err member so we
fall back to manual xmit)

Sebastian
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