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

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Date:	Fri, 11 Nov 2011 13:12:31 +0000
From:	Tabi Timur-B04825 <B04825@...escale.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Tabi Timur-B04825 <B04825@...escale.com>,
	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Baruch Siach <baruch@...s.co.il>,
	Kumar Gala <galak@...nel.crashing.org>,
	Wood Scott-B07421 <B07421@...escale.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	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

Benjamin Herrenschmidt wrote:

> For PHYS_64BIT, I'm not 100% certain at this point what the best
> approach it. It should clearly be fixed and hidden for real 64-bit. As
> for the rest, should it be user selectable ?

I have been unable to convince our BSP teams that the minor performance 
improvement of a 32-bit kernel vs 36-bit is not worth the hassle.  We ship 
a lot of lower-end e500 systems that will never have more than 2GB of memory.

The P1022DS is an example.  We ship 32-bit and 36-bit versions of U-Boot, 
the kernel, and the device tree.  I recently added code to U-boot to 
detect if you have a mismatched U-Boot and device tree, because it is so 
easy to mess that up.  I would like nothing more than to abandon 32-bit 
for all e500, but I don't see Freescale allowing that any time soon.

-- 
Timur Tabi
Linux kernel developer at Freescale
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