[Xen-devel] [PATCH v6 1/4] pci: add pci_iomap_wc() variants

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

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

This allows drivers to take advantage of write-combining
when possible. The PCI specification does not allow for us
to automatically identify a memory region which needs
write-combining so drivers have to identify these areas
on their own. There is IORESOURCE_PREFETCH but as clarified
by Michael and confirmed later by Bjorn, PCI prefetch bit
merely means bridges can combine writes and prefetch reads.
Prefetch does not affect ordering rules and does not allow
writes to be collapsed [0]. WC is stronger, it allows collapsing
and changes ordering rules. WC can also hurt latency as small
writes are buffered. Because of all this drivers needs to
know what they are doing, we can't set a write-combining
preference flag in the pci core automatically for drivers.

Lastly although there is also arch_phys_wc_add() this makes
use of architecture specific write-combining *hacks* and
the only one currently defined and used is MTRR for x86.
MTRRs are legacy, limited in number, have restrictive size
constraints, and are known to interact pooly with the BIOS.
MTRRs should only really be considered on old video framebuffer
drivers. If we made ioremap_wc() and similar calls start
automatically adding MTRRs, then performance will vary wildly
with the order of driver loading because we'll run out of MTRRs
part-way through bootup.

There are a few motivations for phasing out of MTRR and
helping driver change over to use write-combining with PAT:

a) Take advantage of PAT when available

b) Help bury MTRR code away, MTRR is architecture specific and on
   x86 its replaced by PAT

c) Help with the goal of eventually using _PAGE_CACHE_UC over
   _PAGE_CACHE_UC_MINUS on x86 on ioremap_nocache() (see commit
   de33c442e titled "x86 PAT: fix performance drop for glx,
   use UC minus for ioremap(), ioremap_nocache() and
   pci_mmap_page_range()")

[0] https://lkml.org/lkml/2015/4/21/714

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: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
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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