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

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Message-ID: <501ACA94-491F-43A7-9DCF-485BEE824DD5@boeing.com>
Date:	Thu, 10 Nov 2011 10:31:57 -0600
From:	"Moffett, Kyle D" <Kyle.D.Moffett@...ing.com>
To:	Timur Tabi <timur@...escale.com>
CC:	"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>,
	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

On Nov 10, 2011, at 09:04, Timur Tabi wrote:
> 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.

Did you look at the existing code?  It's already that complicated:

config ARCH_PHYS_ADDR_T_64BIT
	def_bool PPC64 || PHYS_64BIT

config ARCH_DMA_ADDR_T_64BIT
	def_bool ARCH_PHYS_ADDR_T_64BIT

config {P1022_DS,P2041_RDB,P3041_DS,P3060_QDS,P4080_DS,P5020_DS}
	select PHYS_64BIT

config 44x
	select PHYS_64BIT

config PTE_64BIT
	bool
	depends on 44x || E500 || PPC_86xx
	default y if PHYS_64BIT

config PHYS_64BIT
	bool 'Large physical address support' if E500 || PPC_86xx
	depends on (44x || E500 || PPC_86xx) && !PPC_83xx && !PPC_82xx

Even worse, PHYS_64BIT is not set on 64-bit processors, but there is
a lot of driver code that seems to assume PHYS_64BIT indicates the
size of "phys_addr_t".

> 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.

The big issue is that the Kconfig docs are very clear that "select"
should not be used on user-visible options (AKA: PHYS_64BIT), and yet
half the PPC_85xx boards have this tidbit:
	select PHYS_64BIT # The DTS has 36-bit addresses

I'm totally OK with removing that from all those boards, but to preserve
the existing behavior (also used by the entire 44x platform) I added the
new config symbol PHYS_64BIT_DT_REQUIRED, which is used to control
whether or not the "PHYS_64BIT" option is even visible to the user.

I originally called it "PHYS_64BIT_REQUIRED", but since all of the
board comments talked about 36-bit DTS addresses, I added the _DT_.


> 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.

If this is true, then why does PHYS_64BIT have that big ugly list of
dependencies right now?  I don't know about the other platforms well
enough to tell what would break by enabling PHYS_64BIT, but I assume
that something in the past caused that dependency list.

Cheers,
Kyle Moffett

--
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I'm keeping a blog here: http://pureperl.blogspot.com/
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