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Date:   Tue, 4 Apr 2017 16:36:31 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     linux-pci@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/17] pci: Add arch_can_pci_mmap_wc() macro

On Wed, Mar 22, 2017 at 01:25:18PM +0000, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.co.uk>
> 
> Most of the almost-identical versions of pci_mmap_page_range() silently
> ignore the 'write_combine' argument and give uncached mappings.
> 
> Yet we allow the PCIIOC_WRITE_COMBINE ioctl in /proc/bus/pci, expose the
> 'resourceX_wc' file in sysfs, and allow an attempted mapping to apparently
> succeed.
> 
> To fix this, introduce a macro arch_can_pci_mmap_wc() which indicates
> whether the platform can do a write-combining mapping. On x86 this ends
> up being pat_enabled(), while the few other platforms that support it
> can just set it to a literal '1'.
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> ---
>  Documentation/filesystems/sysfs-pci.txt |  4 ++++
>  arch/ia64/include/asm/pci.h             |  2 ++
>  arch/powerpc/include/asm/pci.h          |  5 +++--
>  arch/x86/include/asm/pci.h              |  2 ++
>  arch/xtensa/include/asm/pci.h           |  6 +++++-
>  arch/xtensa/kernel/pci.c                |  2 +-
>  drivers/pci/pci-sysfs.c                 |  6 ++++--
>  drivers/pci/proc.c                      | 17 ++++++++++-------
>  8 files changed, 31 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/filesystems/sysfs-pci.txt b/Documentation/filesystems/sysfs-pci.txt
> index 6ea1ced..25b7f1c 100644
> --- a/Documentation/filesystems/sysfs-pci.txt
> +++ b/Documentation/filesystems/sysfs-pci.txt
> @@ -117,6 +117,10 @@ code must define HAVE_PCI_MMAP and provide a pci_mmap_page_range function.
>  Platforms are free to only support subsets of the mmap functionality, but
>  useful return codes should be provided.
>  
> +Platforms which support write-combining maps of PCI resources must define
> +arch_can_pci_mmap_wc() which shall evaluate to non-zero at runtime when
> +write-combining is permitted.
> +
>  Legacy resources are protected by the HAVE_PCI_LEGACY define.  Platforms
>  wishing to support legacy functionality should define it and provide
>  pci_legacy_read, pci_legacy_write and pci_mmap_legacy_page_range functions.
> diff --git a/arch/ia64/include/asm/pci.h b/arch/ia64/include/asm/pci.h
> index c0835b0..6283758 100644
> --- a/arch/ia64/include/asm/pci.h
> +++ b/arch/ia64/include/asm/pci.h
> @@ -51,6 +51,8 @@ extern unsigned long ia64_max_iommu_merge_mask;
>  #define PCI_DMA_BUS_IS_PHYS	(ia64_max_iommu_merge_mask == ~0UL)
>  
>  #define HAVE_PCI_MMAP
> +#define arch_can_pci_mmap_wc()	1
> +
>  extern int pci_mmap_page_range (struct pci_dev *dev, struct vm_area_struct *vma,
>  				enum pci_mmap_state mmap_state, int write_combine);
>  #define HAVE_PCI_LEGACY
> diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
> index 93eded8..b5b68c6 100644
> --- a/arch/powerpc/include/asm/pci.h
> +++ b/arch/powerpc/include/asm/pci.h
> @@ -81,8 +81,9 @@ struct vm_area_struct;
>  int pci_mmap_page_range(struct pci_dev *pdev, struct vm_area_struct *vma,
>  			enum pci_mmap_state mmap_state, int write_combine);
>  
> -/* Tell drivers/pci/proc.c that we have pci_mmap_page_range() */
> -#define HAVE_PCI_MMAP	1
> +/* Tell drivers/pci/proc.c that we have pci_mmap_page_range() and it does WC */
> +#define HAVE_PCI_MMAP		1
> +#define arch_can_pci_mmap_wc()	1
>  
>  extern int pci_legacy_read(struct pci_bus *bus, loff_t port, u32 *val,
>  			   size_t count);
> diff --git a/arch/x86/include/asm/pci.h b/arch/x86/include/asm/pci.h
> index 1411dbe..f6e22c2 100644
> --- a/arch/x86/include/asm/pci.h
> +++ b/arch/x86/include/asm/pci.h
> @@ -7,6 +7,7 @@
>  #include <linux/string.h>
>  #include <linux/scatterlist.h>
>  #include <asm/io.h>
> +#include <asm/pat.h>
>  #include <asm/x86_init.h>
>  
>  #ifdef __KERNEL__
> @@ -102,6 +103,7 @@ int pcibios_set_irq_routing(struct pci_dev *dev, int pin, int irq);
>  
>  
>  #define HAVE_PCI_MMAP
> +#define arch_can_pci_mmap_wc()	pat_enabled()
>  extern int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,
>  			       enum pci_mmap_state mmap_state,
>  			       int write_combine);
> diff --git a/arch/xtensa/include/asm/pci.h b/arch/xtensa/include/asm/pci.h
> index 5d6bd93..f106879 100644
> --- a/arch/xtensa/include/asm/pci.h
> +++ b/arch/xtensa/include/asm/pci.h
> @@ -51,7 +51,11 @@ int pci_mmap_page_range(struct pci_dev *pdev, struct vm_area_struct *vma,
>  			enum pci_mmap_state mmap_state, int write_combine);
>  
>  /* Tell drivers/pci/proc.c that we have pci_mmap_page_range() */
> -#define HAVE_PCI_MMAP	1
> +#define HAVE_PCI_MMAP		1
> +
> +/* This was wrapped in #if 0 since the first merge of xtensa support...
> +#define arch_can_pci_mmap_wc()	1
> +*/
>  
>  #endif /* __KERNEL__ */
>  
> diff --git a/arch/xtensa/kernel/pci.c b/arch/xtensa/kernel/pci.c
> index b848cc3..c5944d3 100644
> --- a/arch/xtensa/kernel/pci.c
> +++ b/arch/xtensa/kernel/pci.c
> @@ -345,7 +345,7 @@ __pci_mmap_set_pgprot(struct pci_dev *dev, struct vm_area_struct *vma,
>  
>  	/* Set to write-through */
>  	prot = (prot & _PAGE_CA_MASK) | _PAGE_CA_WT;
> -#if 0
> +#ifdef arch_can_pci_mmap_wc

This hunk seems like maybe it should be a separate patch.

The rest of the patch:

  - skips creation of /sys/.../resourceX_wc if !arch_can_pci_mmap_wc()
  - makes PCIIOC_WRITE_COMBINE fail if !arch_can_pci_mmap_wc()

This part seems different -- it changes the way pci_mmap_page_range()
works.

>  	if (!write_combine)
>  		prot |= _PAGE_WRITETHRU;
>  #endif
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 7ac258f..cf2c7d8 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -1210,10 +1210,12 @@ static int pci_create_resource_files(struct pci_dev *pdev)
>  			continue;
>  
>  		retval = pci_create_attr(pdev, i, 0);
> +#ifdef arch_can_pci_mmap_wc
>  		/* for prefetchable resources, create a WC mappable file */
> -		if (!retval && pdev->resource[i].flags & IORESOURCE_PREFETCH)
> +		if (!retval && arch_can_pci_mmap_wc() &&
> +		    pdev->resource[i].flags & IORESOURCE_PREFETCH)
>  			retval = pci_create_attr(pdev, i, 1);
> -
> +#endif
>  		if (retval) {
>  			pci_remove_resource_files(pdev);
>  			return retval;
> diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
> index dc8912e..c49be71 100644
> --- a/drivers/pci/proc.c
> +++ b/drivers/pci/proc.c
> @@ -209,15 +209,18 @@ static long proc_bus_pci_ioctl(struct file *file, unsigned int cmd,
>  		fpriv->mmap_state = pci_mmap_mem;
>  		break;
>  
> +#ifdef arch_can_pci_mmap_wc

Can we get rid of these #ifdefs in the code by adding this to linux/pci.h?

  #ifndef arch_can_pci_mmap_wc
  #define arch_can_pci_mmap_wc()	0
  #endif

>  	case PCIIOC_WRITE_COMBINE:
> -		if (arg)
> -			fpriv->write_combine = 1;
> -		else
> -			fpriv->write_combine = 0;
> -		break;
> -
> +		if (arch_can_pci_mmap_wc()) {
> +			if (arg)
> +				fpriv->write_combine = 1;
> +			else
> +				fpriv->write_combine = 0;
> +			break;
> +		}
> +		/* If arch decided it can't, fall through... */
> +#endif /* arch_can_pci_mmap_wc */
>  #endif /* HAVE_PCI_MMAP */
> -
>  	default:
>  		ret = -EINVAL;
>  		break;
> -- 
> 2.9.3
> 

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