linux-kernel - Re: [PATCH v2 00/16] ARM: support for ICP DAS LP-8x4x (with dts)

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Date:	Mon, 16 Dec 2013 18:56:11 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Sergei Ianovich <ynvich@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Daniel Mack <zonque@...il.com>,
	Haojian Zhuang <haojian.zhuang@...il.com>
Subject: Re: [PATCH v2 00/16] ARM: support for ICP DAS LP-8x4x (with dts)

On Monday 16 December 2013, Sergei Ianovich wrote:
> PXA27x memory bus can have up to 10 devices: up to 6 slower
> flash/SRAM/variable-latency-IO selected by nCS<0> to <5>, and up to 4
> partions of SDRAM selected by nSDCS<0> to <3>.
> 
> It appears that the FPGA is directly connected to the memory bus and is
> selected by nCS<5>. According to MSC2 configuration (already in the
> mainstream U-Boot), the FPGA is configured as a synchronous SRAM with
> access cycle of 30x memory bus cycles. So I made it a top-level bus like
> pxabus.
> 
> Ethernet devices are also connected to the memory bus via some kind of
> gate array. This one is a bit faster -- 15x memory cycles, which is
> still a lot. It explains why network transfers are never faster than 15
> Mbit/s.

Ok, I see. This sounds like some of the other platforms we have with
external memory buses. If there is a chance that Linux ever has to
program the per-CS settings into the bus controller, I would suggest
to represent that as well as a separate node, like this

extbus {
	compatible = "simple-bus";

	#address-cells = <2>; /* first cell is nCS, second is address */
	#size-cells = <1>;

	ranges = <0 0 0 0x04000000>
		 <1 0 0x04000000 0x04000000>
		 <2 0 0x08000000 0x04000000>
		 <3 0 0x0c000000 0x04000000>
	  	 <4 0 0x10000000 0x04000000>
	  	 <5 0 0x14000000 0x04000000>;

	timings = ...;

	flash@0 {
		reg = <0 0x0 0x02000000>;
		...
	};

	flash@1 {
		reg = <1 0x0 0x02000000>;
		...
	};

	fpga@5 {
		#address-cells = <1>;
		#size-cells = <1>;
		ranges = <0 5 0x03000000 0x10000>;
		...
	};
};

In case there is a driver for the extbus node, I would not make it
"simple-bus" but instead add a separate compatible string to match
the driver, and let that driver call of_platform_populate
to probe the children after the bus is set up.

	Arnd
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