[7/8] 8xx: Restore _PAGE_WRITETHRU

8xx has not had WRITETHRU due to lack of bits in the pte.
After the recent rewrite of the 8xx TLB code, there are
two bits left. Use one of them to WRITETHRU.

Perhaps use the last SW bit to PAGE_SPECIAL or PAGE_FILE?
---
 arch/powerpc/include/asm/pte-8xx.h |    5 +++--
 arch/powerpc/kernel/head_8xx.S     |    8 ++++++++
 2 files changed, 11 insertions(+), 2 deletions(-)

On Sun, 2009-10-11 at 18:35 +0200, Joakim Tjernlund wrote:
> 8xx has not had WRITETHRU due to lack of bits in the pte.
> After the recent rewrite of the 8xx TLB code, there are
> two bits left. Use one of them to WRITETHRU.
> 
> Perhaps use the last SW bit to PAGE_SPECIAL or PAGE_FILE?

_PAGE_FILE can already overwrite other bits as it's only set
when !present, and should pretty much always be 0x2

I think I've replaced _PAGE_EXEC with _PAGE_SPECIAL already
upstream since _PAGE_EXEC is unused on 8xx.

Cheers,
Ben.

> ---
>  arch/powerpc/include/asm/pte-8xx.h |    5 +++--
>  arch/powerpc/kernel/head_8xx.S     |    8 ++++++++
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pte-8xx.h b/arch/powerpc/include/asm/pte-8xx.h
> index f23cd15..9349d83 100644
> --- a/arch/powerpc/include/asm/pte-8xx.h
> +++ b/arch/powerpc/include/asm/pte-8xx.h
> @@ -34,12 +34,13 @@
>  #define _PAGE_SHARED	0x0004	/* No ASID (context) compare */
>  #define _PAGE_DIRTY	0x0100	/* C: page changed */
>  
> -/* These 3 software bits must be masked out when the entry is loaded
> - * into the TLB, 2 SW bits left.
> +/* These 4 software bits must be masked out when the entry is loaded
> + * into the TLB, 1 SW bit left(0x0080).
>   */
>  #define _PAGE_EXEC	0x0008	/* software: i-cache coherency required */
>  #define _PAGE_GUARDED	0x0010	/* software: guarded access */
>  #define _PAGE_ACCESSED	0x0020	/* software: page referenced */
> +#define _PAGE_WRITETHRU	0x0040	/* software: caching is write through */
>  
>  /* Setting any bits in the nibble with the follow two controls will
>   * require a TLB exception handler change.  It is assumed unused bits
> diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
> index 371b606..db5207e 100644
> --- a/arch/powerpc/kernel/head_8xx.S
> +++ b/arch/powerpc/kernel/head_8xx.S
> @@ -422,6 +422,10 @@ DataStoreTLBMiss:
>  	 * above.
>  	 */
>  	rlwimi	r11, r10, 0, 27, 27
> +	/* Insert the WriteThru flag into the TWC from the Linux PTE.
> +	 * It is bit 25 in the Linux PTE and bit 30 in the TWC
> +	 */
> +	rlwimi	r11, r10, 32-5, 30, 30
>  	DO_8xx_CPU6(0x3b80, r3)
>  	mtspr	SPRN_MD_TWC, r11
>  
> @@ -559,6 +563,10 @@ DARFix:	/* Return from dcbx instruction bug workaround, r10 holds value of DAR *
>  	 * It is bit 27 of both the Linux PTE and the TWC
>  	 */
>  	rlwimi	r11, r10, 0, 27, 27
> +	/* Insert the WriteThru flag into the TWC from the Linux PTE.
> +	 * It is bit 25 in the Linux PTE and bit 30 in the TWC
> +	 */
> +	rlwimi	r11, r10, 32-5, 30, 30
>  	DO_8xx_CPU6(0x3b80, r3)
>  	mtspr	SPRN_MD_TWC, r11
>  	mfspr	r11, SPRN_MD_TWC	/* get the pte address again */

Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote on 11/10/2009 23:26:27:
>
> On Sun, 2009-10-11 at 18:35 +0200, Joakim Tjernlund wrote:
> > 8xx has not had WRITETHRU due to lack of bits in the pte.
> > After the recent rewrite of the 8xx TLB code, there are
> > two bits left. Use one of them to WRITETHRU.
> >
> > Perhaps use the last SW bit to PAGE_SPECIAL or PAGE_FILE?
>
> _PAGE_FILE can already overwrite other bits as it's only set
> when !present, and should pretty much always be 0x2

OK.

>
> I think I've replaced _PAGE_EXEC with _PAGE_SPECIAL already
> upstream since _PAGE_EXEC is unused on 8xx.

What is page SPECIAL anyway?

On Mon, 2009-10-12 at 00:21 +0200, Joakim Tjernlund wrote:
> > I think I've replaced _PAGE_EXEC with _PAGE_SPECIAL already
> > upstream since _PAGE_EXEC is unused on 8xx.
> 
> What is page SPECIAL anyway? 

It's used on newer kernels to indicates PTEs that map something that
isn't backed by a struct page (ie, not memory)

Cheers,
Ben.