[Xen-devel] [PATCH v8 5/9] PCI: Add pci_iomap_wc() variants

["Luis R. Rodriguez" <mcgrof@xxxxxxxxxxxxxxxx>]

From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>

PCI BARs tell us whether prefetching is safe, but they don't say anything
about write combining (WC).  WC changes ordering rules and allows writes to
be collapsed, so it's not safe in general to use it on a prefetchable
region.

Add pci_iomap_wc() and pci_iomap_wc_range() so drivers can take advantage
of write combining when they know it's safe.

On architectures that don't fully support WC, e.g., x86 without PAT,
drivers for legacy framebuffers may get some of the benefit by using
arch_phys_wc_add() in addition to pci_iomap_wc().  But arch_phys_wc_add()
is unreliable and should be avoided in general.  On x86, it uses MTRRs,
which are limited in number and size, so the results will vary based on
driver loading order.

The goals of adding pci_iomap_wc() are to:

- Give drivers an architecture-independent way to use WC so they can stop
  using interfaces like mtrr_add() (on x86, pci_iomap_wc() uses
  PAT when available)

- Move toward using _PAGE_CACHE_MODE_UC, not _PAGE_CACHE_MODE_UC_MINUS,
  on x86 on ioremap_nocache() (see de33c442ed2a ("x86 PAT: fix
  performance drop for glx, use UC minus for ioremap(), ioremap_nocache()
  and pci_mmap_page_range()")

Link: 
http://lkml.kernel.org/r/1426893517-2511-6-git-send-email-mcgrof@xxxxxxxxxxxxxxxx
Original-posting: 
http://lkml.kernel.org/r/1432163293-20965-1-git-send-email-mcgrof@xxxxxxxxxxxxxxxx
Cc: Toshi Kani <toshi.kani@xxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Suresh Siddha <sbsiddha@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
Cc: Antonino Daplas <adaplas@xxxxxxxxx>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@xxxxxxxxxxxx>
Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Cc: venkatesh.pallipadi@xxxxxxxxx
Cc: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
Cc: Ville SyrjÃlà <syrjala@xxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: konrad.wilk@xxxxxxxxxx
Cc: ville.syrjala@xxxxxxxxxxxxxxx
Cc: david.vrabel@xxxxxxxxxx
Cc: jbeulich@xxxxxxxx
Cc: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
Cc: linux-fbdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
---
 include/asm-generic/pci_iomap.h | 14 ++++++++++
 lib/pci_iomap.c                 | 61 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/include/asm-generic/pci_iomap.h b/include/asm-generic/pci_iomap.h
index 7389c87..b1e17fc 100644
--- a/include/asm-generic/pci_iomap.h
+++ b/include/asm-generic/pci_iomap.h
@@ -15,9 +15,13 @@ struct pci_dev;
 #ifdef CONFIG_PCI
 /* Create a virtual mapping cookie for a PCI BAR (memory or IO) */
 extern void __iomem *pci_iomap(struct pci_dev *dev, int bar, unsigned long 
max);
+extern void __iomem *pci_iomap_wc(struct pci_dev *dev, int bar, unsigned long 
max);
 extern void __iomem *pci_iomap_range(struct pci_dev *dev, int bar,
                                     unsigned long offset,
                                     unsigned long maxlen);
+extern void __iomem *pci_iomap_wc_range(struct pci_dev *dev, int bar,
+                                       unsigned long offset,
+                                       unsigned long maxlen);
 /* Create a virtual mapping cookie for a port on a given PCI device.
  * Do not call this directly, it exists to make it easier for architectures
  * to override */
@@ -34,12 +38,22 @@ static inline void __iomem *pci_iomap(struct pci_dev *dev, 
int bar, unsigned lon
        return NULL;
 }
 
+static inline void __iomem *pci_iomap_wc(struct pci_dev *dev, int bar, 
unsigned long max)
+{
+       return NULL;
+}
 static inline void __iomem *pci_iomap_range(struct pci_dev *dev, int bar,
                                            unsigned long offset,
                                            unsigned long maxlen)
 {
        return NULL;
 }
+static inline void __iomem *pci_iomap_wc_range(struct pci_dev *dev, int bar,
+                                              unsigned long offset,
+                                              unsigned long maxlen)
+{
+       return NULL;
+}
 #endif
 
 #endif /* __ASM_GENERIC_IO_H */
diff --git a/lib/pci_iomap.c b/lib/pci_iomap.c
index bcce5f1..9604dcb 100644
--- a/lib/pci_iomap.c
+++ b/lib/pci_iomap.c
@@ -52,6 +52,46 @@ void __iomem *pci_iomap_range(struct pci_dev *dev,
 EXPORT_SYMBOL(pci_iomap_range);
 
 /**
+ * pci_iomap_wc_range - create a virtual WC mapping cookie for a PCI BAR
+ * @dev: PCI device that owns the BAR
+ * @bar: BAR number
+ * @offset: map memory at the given offset in BAR
+ * @maxlen: max length of the memory to map
+ *
+ * Using this function you will get a __iomem address to your device BAR.
+ * You can access it using ioread*() and iowrite*(). These functions hide
+ * the details if this is a MMIO or PIO address space and will just do what
+ * you expect from them in the correct way. When possible write combining
+ * is used.
+ *
+ * @maxlen specifies the maximum length to map. If you want to get access to
+ * the complete BAR from offset to the end, pass %0 here.
+ * */
+void __iomem *pci_iomap_wc_range(struct pci_dev *dev,
+                                int bar,
+                                unsigned long offset,
+                                unsigned long maxlen)
+{
+       resource_size_t start = pci_resource_start(dev, bar);
+       resource_size_t len = pci_resource_len(dev, bar);
+       unsigned long flags = pci_resource_flags(dev, bar);
+
+       if (len <= offset || !start)
+               return NULL;
+       len -= offset;
+       start += offset;
+       if (maxlen && len > maxlen)
+               len = maxlen;
+       if (flags & IORESOURCE_IO)
+               return NULL;
+       if (flags & IORESOURCE_MEM)
+               return ioremap_wc(start, len);
+       /* What? */
+       return NULL;
+}
+EXPORT_SYMBOL_GPL(pci_iomap_wc_range);
+
+/**
  * pci_iomap - create a virtual mapping cookie for a PCI BAR
  * @dev: PCI device that owns the BAR
  * @bar: BAR number
@@ -70,4 +110,25 @@ void __iomem *pci_iomap(struct pci_dev *dev, int bar, 
unsigned long maxlen)
        return pci_iomap_range(dev, bar, 0, maxlen);
 }
 EXPORT_SYMBOL(pci_iomap);
+
+/**
+ * pci_iomap_wc - create a virtual WC mapping cookie for a PCI BAR
+ * @dev: PCI device that owns the BAR
+ * @bar: BAR number
+ * @maxlen: length of the memory to map
+ *
+ * Using this function you will get a __iomem address to your device BAR.
+ * You can access it using ioread*() and iowrite*(). These functions hide
+ * the details if this is a MMIO or PIO address space and will just do what
+ * you expect from them in the correct way. When possible write combining
+ * is used.
+ *
+ * @maxlen specifies the maximum length to map. If you want to get access to
+ * the complete BAR without checking for its length first, pass %0 here.
+ * */
+void __iomem *pci_iomap_wc(struct pci_dev *dev, int bar, unsigned long maxlen)
+{
+       return pci_iomap_wc_range(dev, bar, 0, maxlen);
+}
+EXPORT_SYMBOL_GPL(pci_iomap_wc);
 #endif /* CONFIG_PCI */
-- 
2.3.2.209.gd67f9d5.dirty


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