linux-kernel - Re: RFC: running out of x86 boot loader IDs

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Message-ID: <4982398D.1020506@zytor.com>
Date:	Thu, 29 Jan 2009 15:19:41 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: RFC: running out of x86 boot loader IDs

Ingo Molnar wrote:
>>
>> - If the boot loader ID is E, the current pad1 field at 0x226 is  
>> repurposed as an extended loader ID.  The reason to use the pad1 field  
>> is that it is present in all headers since version 2.02.  The boot  
>> loader ID will simply be: ((extended ID + 0x10) << 4) + (version), where  
>> (version) as before is (type_of_loader & 15).  This is the value which  
>> will be reported in /proc/sys/kernel/bootloader_type.
>>
>> The biggest question is probably: is there a need/desire for an extended 
>> version field, or is four bits enough for existing bootloader needs?
> 
> i think it's prudent to add an extension mechanism, regardless of demand. 
> Existing bootloader projects will be content with the IDs they already 
> have so they are unlikely to request new ones. Future bootloader projects 
> cannot request it because they dont exist yet. So there's no-one to talk 
> up.
> 

Good point.  If so, it probably makes most sense to split pad1 into two 
one-byte fields; one for ID and one for version.  The bootloader_type 
will have to continue to be (ID << 4)+basic_version; we can presumably 
add a bootloader_version file which can report a bigger ID.

Either that or we can expose the raw pad1 field as bootloader_exttype 
and force any users to be aware of the field splits.

	-hpa

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