Modern MIPS cores (like P5600/6600, M5150/6520, end so on) which got L2-cache on chip also can enable a special type Cache-Coherency attribute (CCA) named UnCached Accelerated attribute (UCA). In this way uncached accelerated accesses are treated the same way as non-accelerated uncached accesses, but uncached stores are gathered together for more efficient bus utilization. So to speak this CCA enables uncached transactions to better utilize bus bandwidth via burst transactions.
This is exactly why ioremap_wc() method has been introduced in linux. Alas MIPS-platform code hasn't implemented it so far, instead default one has been used which was an alias to ioremap_nocache. In order to fix this we added MIPS-specific ioremap_wc() macro substituted by generic __ioremap_mode() method call with writecombine CPU-info field passed. It shall create real ioremap_wc() method if CPU-cache supports UCA feature and fall-back to _CACHE_UNCACHED attribute if one doesn't. Additionally platform-specific io.h shall declare ARCH_HAS_IOREMAP_WC macro as indication of architectural definition of ioremap_wc() (similar to x86/powerpc).
Signed-off-by: Serge Semin fancer.lancer@gmail.com Singed-off-by: Paul Burton paul.burton@mips.com Cc: James Hogan jhogan@kernel.org Cc: Ralf Baechle ralf@linux-mips.org Cc: linux-mips@linux-mips.org Cc: stable@vger.kernel.org --- arch/mips/include/asm/io.h | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)
diff --git a/arch/mips/include/asm/io.h b/arch/mips/include/asm/io.h index 4d709b61d..d4f8cdc58 100644 --- a/arch/mips/include/asm/io.h +++ b/arch/mips/include/asm/io.h @@ -12,6 +12,8 @@ #ifndef _ASM_IO_H #define _ASM_IO_H
+#define ARCH_HAS_IOREMAP_WC + #include <linux/compiler.h> #include <linux/kernel.h> #include <linux/types.h> @@ -278,6 +280,27 @@ static inline void __iomem * __ioremap_mode(phys_addr_t offset, unsigned long si #define ioremap_cache ioremap_cachable
/* + * ioremap_wc - map bus memory into CPU space + * @offset: bus address of the memory + * @size: size of the resource to map + * + * ioremap_wc performs a platform specific sequence of operations to + * make bus memory CPU accessible via the readb/readw/readl/writeb/ + * writew/writel functions and the other mmio helpers. The returned + * address is not guaranteed to be usable directly as a virtual + * address. + * + * This version of ioremap ensures that the memory is marked uncachable + * but accelerated by means of write-combining feature. It is specifically + * useful for PCIe prefetchable windows, which may vastly improve a + * communications performance. If it was determined on boot stage, what + * CPU CCA doesn't support UCA, the method shall fall-back to the + * _CACHE_UNCACHED option (see cpu_probe() method). + */ +#define ioremap_wc(offset, size) \ + __ioremap_mode((offset), (size), boot_cpu_data.writecombine) + +/* * These two are MIPS specific ioremap variant. ioremap_cacheable_cow * requests a cachable mapping, ioremap_uncached_accelerated requests a * mapping using the uncached accelerated mode which isn't supported on