linux-kernel - Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

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Date:	Thu, 21 Aug 2014 10:34:21 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Tony Lindgren <tony@...mide.com>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	balbi@...com, Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

On 08/15/2014 11:02 PM, Tony Lindgren wrote:
> * Sebastian Andrzej Siewior <bigeasy@...utronix.de> [140815 11:13]:
>> +#ifdef CONFIG_SERIAL_8250_DMA
>> +	if (pdev->dev.of_node) {
>> +		/*
>> +		 * Oh DMA support. If there are no DMA properties in the DT then
>> +		 * we will fall back to a generic DMA channel which does not
>> +		 * really work here. To ensure that we do not get a generic DMA
>> +		 * channel assigned, we have the the_no_dma_filter_fn() here.
>> +		 * To avoid "failed to request DMA" messages we check for DMA
>> +		 * properties in DT.
>> +		 */
>> +		ret = of_property_count_strings(pdev->dev.of_node, "dma-names");
>> +		if (ret == 2) {
>> +			up.dma = &priv->omap8250_dma;
>> +			priv->omap8250_dma.fn = the_no_dma_filter_fn;
>> +			priv->omap8250_dma.rx_size = RX_TRIGGER;
>> +			priv->omap8250_dma.rxconf.src_maxburst = RX_TRIGGER;
>> +			priv->omap8250_dma.txconf.dst_maxburst = TX_TRIGGER;
>> +
>> +			if (of_machine_is_compatible("ti,am33xx"))
>> +				up.bugs |= UART_BUG_DMATX;
>> +		}
>> +	}
>> +#endif
> 
> Hmm I wonder if there's a more generic solution to this?

To what exactly? The trigger level, the TX-bug or using DMA in the
first place?

> It would be also nice to be able to specify the use of DMA from
> the board specific .dts file.

Really? I don't see this requirement for MMC for instance. However you
still could provide an empty "dma-names" property in your board .dts
file if you do not want use DMA. Would this work?

> Also, with DMA enabled, looks like omap deeper idle states are
> blocked as the DMA stays reserved. After I commented out the
> DMA info for my console UART, PM started working.

Hmm. This would explain something. This would mean that I should cancel
the RX DMA transfer in the PM-suspend routine. Let me see how that
works.

> Regards,
> 
> Tony

Sebastian
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