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

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Message-ID: <20140821184416.GF10066@atomide.com>
Date:	Thu, 21 Aug 2014 11:44:16 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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

* Sebastian Andrzej Siewior <bigeasy@...utronix.de> [140821 01:37]:
> 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?

Oh sorry, I meant to having to do of_property_count_strings to
figure out if it's correct or not. 
 
> > 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?

OK yeah that works. And that's needed mostly because of the issue
below.
 
> > 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.

OK and if the DMA works with PM, then I don't see why we would not
want to have it automatically enabled.

BTW, looks like the ports move around now though. If set a port
to disabled with status = "disabled"; in the .dts file, you'll get
a different console which does not happen with omap-serial I believe.

Regards,

Tony
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