linux-kernel - Re: [RFC PATCH 02/17] powerpc: Split up PHYS_64BIT config option to fix "select" issues

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Date:	Thu, 10 Nov 2011 08:04:15 -0600
From:	Timur Tabi <timur@...escale.com>
To:	Kyle Moffett <Kyle.D.Moffett@...ing.com>
CC:	<linuxppc-dev@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>,
	Baruch Siach <baruch@...s.co.il>,
	Kumar Gala <galak@...nel.crashing.org>,
	Scott Wood <scottwood@...escale.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Wolfram Sang <w.sang@...gutronix.de>,
	Grant Likely <grant.likely@...retlab.ca>,
	Anatolij Gustschin <agust@...x.de>,
	Paul Bolle <pebolle@...cali.nl>,
	Sonny Rao <sonnyrao@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 02/17] powerpc: Split up PHYS_64BIT config option
 to fix "select" issues

Kyle Moffett wrote:
>   CONFIG_PHYS_64BIT_SUPPORTED:
>     This hidden option should be selected by any CPU type which supports
>     64-bit physical addresses.  This will enable the PHYS_64BIT option
>     to be selected.  It is (obviously) always set on PPC64.
> 
>   CONFIG_PHYS_64BIT_DT_REQUIRED:
>     This hidden option should be selected by any board or platform which
>     has >32-bit physical devices present in hardware.  If this is set
>     then the CONFIG_PHYS_64BIT option will be forcibly enabled and
>     hidden from the user.  It is (obviously) always set on PPC64.
> 
>   CONFIG_PHYS_64BIT:
>     This option is user-controllable, where allowed by CPU and platform
>     settings, and should never be pointed at with a "select" statement.
>     Due to the values of the above two options, this is never visible on
>     PPC64.

I'm with Kumar on this.  I don't see the point of making it so complicated.  No kernel should ever have to care with a DT is 64-bit or 32-bit.  If you build a kernel with 64-bit phys support, then it will work with any DT.

U-Boot already warns you if the DT and the actual physical addresses of the devices don't match.

There are only two reasons to create a 32-bit kernel:

1) A small performance improvement on systems with 2GB or less.
2) Some SOC devices only support 32-bit physical addresses, so the easiest way to ensure a compatible address is to prohibit memory above 4GB.

Item #2 is not something that we really worry about.  It's only a real problem on true 64-bit kernels, and in those cases, we cannot build a kernel with 32-bit phys support, so the driver needs to handle it internally.

-- 
Timur Tabi
Linux kernel developer at Freescale

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