[PATCH -V5 13/13] arch/powerpc: Update VSID allocation documentation

[Aneesh Kumar K.V]

Paul Mackerras <paulus at samba.org> writes:

> On Mon, Jul 30, 2012 at 04:52:19PM +0530, Aneesh Kumar K.V wrote:
>> From: "Aneesh Kumar K.V" <aneesh.kumar at linux.vnet.ibm.com>
>> 
>> This update the proto-VSID and VSID scramble related information
>> to be more generic by using names instead of current values.
>
> Comments below...
>
>> - * VSID allocation
>> + * VSID allocation (256MB segment)
>>   *
>> - * We first generate a 36-bit "proto-VSID".  For kernel addresses this
>> - * is equal to the ESID, for user addresses it is:
>> - *	(context << 15) | (esid & 0x7fff)
>> + * We first generate a 38-bit "proto-VSID".  For kernel addresses this
>> + * is equal to the ESID | 1 << 37, for user addresses it is:
>> + *	(context << USER_ESID_BITS) | (esid & (1U << USER_ESID_BITS))
> 					      ^^^^^^^^^^^^^^^^^^^^^^
> should be ((1U << USER_ESID_BITS) - 1)
>
>>   *
>> - * The two forms are distinguishable because the top bit is 0 for user
>> - * addresses, whereas the top two bits are 1 for kernel addresses.
>> - * Proto-VSIDs with the top two bits equal to 0b10 are reserved for
>> - * now.
>> + * This splits the proto-VSID into the below range
>> + *  0 - (2^(CONTEXT_BITS + USER_ESID_BITS) - 1) : User proto-VSID range
>> + *  2^(CONTEXT_BITS + USER_ESID_BITS) - 2^(VSID_BITS) : Kernel proto-VSID range
>
> Perhaps point out also that CONTEXT_BITS + USER_ESID_BITS == VSID_BITS - 1,
> that is, you have assigned half of the space to user processes and half
> to the kernel.
>

updated

>> -/*
>> - * WARNING - If you change these you must make sure the asm
>> - * implementations in slb_allocate (slb_low.S), do_stab_bolted
>> - * (head.S) and ASM_VSID_SCRAMBLE (below) are changed accordingly.
>> - */
>
> Are you absolutely sure that nothing in the assembly code would need
> to be changed if someone changed these definitions?
>

As a part of this patchset, i didn't touch any of these and the pathcset
do change these values. 

-aneesh