linux-kernel - Re: [PATCH] serial: tegra: add serial driver

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Message-Id: <20121219130307.B8FCB3E0AD7@localhost>
Date:	Wed, 19 Dec 2012 13:03:05 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Mitch Bradley <wmb@...mworks.com>,
	Stephen Warren <swarren@...dotorg.org>
Cc:	Laxman Dewangan <ldewangan@...dia.com>, rob.herring@...xeda.com,
	linux-doc@...r.kernel.org, gregkh@...uxfoundation.org,
	devicetree-discuss@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org, linux-tegra@...r.kernel.org,
	jslaby@...e.cz, alan@...ux.intel.com
Subject: Re: [PATCH] serial: tegra: add serial driver

On Mon, 17 Dec 2012 12:17:16 -1000, Mitch Bradley <wmb@...mworks.com> wrote:
> On 12/17/2012 12:04 PM, Stephen Warren wrote:
> > On 12/17/2012 02:58 PM, Mitch Bradley wrote:
> >> On 12/17/2012 11:36 AM, Stephen Warren wrote:
> >>> On 12/17/2012 05:10 AM, Laxman Dewangan wrote:
> >>>> Nvidia's Tegra has multiple uart controller which supports:
> >>>> - APB dma based controller fifo read/write.
> >>>> - End Of Data interrupt in incoming data to know whether end
> >>>>   of frame achieve or not.
> >>>> - Hw controlled RTS and CTS flow control to reduce SW overhead.
> >>>
> >>>> diff --git a/Documentation/devicetree/bindings/serial/nvidia,serial-tegra.txt b/Documentation/devicetree/bindings/serial/nvidia,serial-tegra.txt
> >>>
> >>>> +NVIDIA Tegra20/Tegra30 high speed (dma based) UART controller driver.
> >>>> +
> >>>> +Required properties:
> >>>> +- compatible : should be "nvidia,tegra20-hsuart", "nvidia,tegra30-hsuart".
> >>>
> >>> One question that isn't addressed here is:
> >>>
> >>> Tegra has 5 UARTs. All of them can use the existing 8250.c by specifying
> >>> compatible = "nvidia,tegra20-uart".
> >>
> >> The way it is supposed to work is that the compatible property should
> >> list "nvidia,tegra30-hsuart" first, followed by a fallback name that
> >> refers to the generic 8250 compatibility.  Having the 8250.c driver bind
> >> to the more-specific tegra30-hsuart name is wrong.
> > 
> > 8250.c binds to nvidia,tegra20-uart, so that aspect is fine.
> > 
> > However, the real issue is that we probably want 4 of the 5 ports to use
> > the plain old 8250.c (so as not to use up too many DMA channels), but
> > just 1 of the ports to use the DMA-capable high-performance driver (e.g.
> > the one that a particular board has hooked up to a Bluetooth radio). The
> > only way to do that with DT that I know of would be to specify different
> > subsets of legal compatible values for each UART in the per-board .dts file.
> 
> 
> That's an okay way to do it.  The whole purpose of the compatible
> property is to support driver binding.  The mantra to "describe the
> hardware" is good, but shouldn't be taken to extremes.
> 
> It would be a good idea to comment the .dts file to explain why the
> compatible property differs between the otherwise-identical nodes.

+1

g.

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