RE: [PATCH 2/2] ARM: dts: r7s9210-rza2mevb: Add support for RZ/A2M EVB

From: Chris Brandt <Chris.Brandt@renesas.com>
To: Rob Herring <robh@kernel.org>
Cc: Simon Horman <horms@verge.net.au>,
	Mark Rutland <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-renesas-soc@vger.kernel.org"
	<linux-renesas-soc@vger.kernel.org>
Subject: RE: [PATCH 2/2] ARM: dts: r7s9210-rza2mevb: Add support for RZ/A2M EVB
Date: Wed, 12 Dec 2018 13:58:59 +0000	[thread overview]
Message-ID: <TYXPR01MB1568DD386FE18BD1F985E5178AA70@TYXPR01MB1568.jpnprd01.prod.outlook.com> (raw)
In-Reply-To: <20181212021549.GA26333@bogus>

On Tuesday, December 11, 2018, Rob Herring wrote:
> > +	/* Cramfs XIP File System in QSPI */
> > +	qspi@20000000 {
> 
> 
> > +		compatible = "mtd-rom";
> 
> This should be the actual Quad-SPI controller and then a flash device
> underneath it. It may work as-is initially, but eventually won't you
> need the flash details?

Not really.
The "QSPI" actually works as a memory mapped SPI (as in, the CPU sees 
the QSPI flash as a linear read-only address range).
Basically, u-boot sets up the QSPI to look like a ROM area, and that's 
it. Everything is running directly from that QSPI flash, so you can't 
make any modifications to it while running, otherwise the whole system will
go down. Therefore, there is no need to know anything about the actually
Flash that is connected, because you are never allowed to interact with it
(or even query it on boot).

This board in particular only has 8MB of RAM. So, it really only works 
well as an XIP system (meaning both kernel and file system are XIP from 
Flash).

Chris