Re: [PATCH] powerpc/8xx: Fix clearing of bits 20-23 in ITLB miss

[Leonardo Bras]

On Tue, 2020-02-11 at 01:28 -0300, Leonardo Bras wrote:
> Looks a valid change.
> rlwimi  r10, r10, 0, 0x0f00 means: 
> r10 = ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) which ends up being
> r10 = r10 
> 
> On ISA, rlwinm is recommended for clearing high order bits.
> rlwinm  r10, r10, 0, ~0x0f00 means:
> r10 = (r10 << 0) & ~0x0f00
> 
> Which does exactly what the comments suggests.
> 
> FWIW:
> Reviwed-by: Leonardo Bras <leonardo@xxxxxxxxxxxxx>


Sorry, I just realized the above was not very clear on my part.

What I meant to say was:
I think your change is correct, as it correctly fixes this line.

I would suggest adding the text bellow to your commit message, making
it easier to understand why rlwimi is not the right instruction clear
bytes 20-23, and why rlwinm is.

The current instruction can be translated to C as:
rlwimi  r10, r10, 0, 0x0f00
r10 = ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) 	->
r10 = (r10 & 0x0f00) | (r10 & ~0x0f00)		->
r10 = r10

The new proposed instruction can be translated to C as:
rlwinm  r10, r10, 0, ~0x0f00 	->
r10 = (r10 << 0) & ~0x0f00

Which clears bits 20-23 as comment on code states.

Best regards,

Leonardo Bras

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