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

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Date:	Mon, 17 Dec 2012 15:04:04 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Mitch Bradley <wmb@...mworks.com>
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 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.
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