Subject: Re: [PATCH V2] powerpc/infiniband: Use cache inhibitted and guarded mapping on powerpc

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 22 Apr 2016 08:47:56 +1000

On Wed, 2016-04-20 at 03:58 -0400, Aneesh Kumar K.V wrote:
> The driver was requesting for a writethrough mapping. But with thoses
> flags we will end up with a SAO mapping because we now have memory
> conherence always enabled. ie, the existing mapping will end up with
> a WIMG value 0b1110 which is Strong Access Order.
> 
> Update this to use cache inhibitted guarded mapping

Why guarded ? If it's performance sensitive (and the driver has
appropriate barriers where needed), you will get write combining
without guarded, you won't with it.

Cheers,
Ben.

> Cc: Mike Marciniszyn <[email-masked]>
> Cc: Doug Ledford <[email-masked]>
> Cc: Sean Hefty <[email-masked]>
> Cc: Hal Rosenstock <[email-masked]>
> Cc: [email-masked]
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
> Changes from v1:
> * resend because V1 never reached mailing list.
> 
> NOTE: This is only compile tested and I am also not sure why ppc64
> needs special handling.
> We need this patch because the series at http://mid.gmane.org/1460182
> [email-masked]
> will drop _PAGE_WRITETHRU
> 
>  drivers/infiniband/hw/qib/qib_file_ops.c | 5 +----
>  drivers/infiniband/hw/qib/qib_pcie.c     | 6 ------
>  2 files changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/drivers/infiniband/hw/qib/qib_file_ops.c
> b/drivers/infiniband/hw/qib/qib_file_ops.c
> index e449e394963f..a3d593f546ec 100644
> --- a/drivers/infiniband/hw/qib/qib_file_ops.c
> +++ b/drivers/infiniband/hw/qib/qib_file_ops.c
> @@ -822,10 +822,7 @@ static int mmap_piobufs(struct vm_area_struct
> *vma,
>  	phys = dd->physaddr + piobufs;
>  
>  #if defined(__powerpc__)
> -	/* There isn't a generic way to specify writethrough
> mappings */
> -	pgprot_val(vma->vm_page_prot) |= _PAGE_NO_CACHE;
> -	pgprot_val(vma->vm_page_prot) |= _PAGE_WRITETHRU;
> -	pgprot_val(vma->vm_page_prot) &= ~_PAGE_GUARDED;
> +	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>  #endif
>  
>  	/*
> diff --git a/drivers/infiniband/hw/qib/qib_pcie.c
> b/drivers/infiniband/hw/qib/qib_pcie.c
> index 4758a3801ae8..6abe1c621aa4 100644
> --- a/drivers/infiniband/hw/qib/qib_pcie.c
> +++ b/drivers/infiniband/hw/qib/qib_pcie.c
> @@ -144,13 +144,7 @@ int qib_pcie_ddinit(struct qib_devdata *dd,
> struct pci_dev *pdev,
>  	addr = pci_resource_start(pdev, 0);
>  	len = pci_resource_len(pdev, 0);
>  
> -#if defined(__powerpc__)
> -	/* There isn't a generic way to specify writethrough
> mappings */
> -	dd->kregbase = __ioremap(addr, len, _PAGE_NO_CACHE |
> _PAGE_WRITETHRU);
> -#else
>  	dd->kregbase = ioremap_nocache(addr, len);
> -#endif
> -
>  	if (!dd->kregbase)
>  		return -ENOMEM;
>