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

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Date:	Mon, 17 Dec 2012 12:17:16 -1000
From:	Mitch Bradley <wmb@...mworks.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	Laxman Dewangan <ldewangan@...dia.com>, grant.likely@...retlab.ca,
	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 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.

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