[PATCH v2 2/2] alpha: enable contiguous memory allocation for coherent DMA

[Magnus Lindholm <linmag7@gmail.com>]

Select HAVE_DMA_CONTIGUOUS and reserve a default CMA area after the
machine has initialized its DMA windows. Prefer top-down placement
below the direct-map and usable RAM limits. Skip the default reservation
when no direct-map window exists. Explicit cma= ranges retain their
generic override semantics; top-down placement is a preference, not a
hard lower bound excluding the first 16 MiB.

Use dma_alloc_contiguous(), which rejects non-blocking callers, before
falling back to alloc_pages_exact(). Pair both paths with
dma_free_contiguous(), whose buddy fallback uses free_pages_exact().
Skip CMA on the GFP_DMA retry after a mapping failure so it cannot
return the same unsuitable CMA pages. Retain zeroing for CMA memory.

With CONFIG_DMA_CMA=y, the default CMA_SIZE_MBYTES setting reserves
16 MiB without a cma= argument. cma=0 disables the default area.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 .../features/io/dma-contiguous/arch-support.txt   |  2 +-
 arch/alpha/Kconfig                                |  1 +
 arch/alpha/kernel/pci_iommu.c                     | 13 +++++++++----
 arch/alpha/kernel/setup.c                         | 15 +++++++++++++++
 4 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/Documentation/features/io/dma-contiguous/arch-support.txt b/Documentation/features/io/dma-contiguous/arch-support.txt
index 3c6ce35d704f..6cd205a991f6 100644
--- a/Documentation/features/io/dma-contiguous/arch-support.txt
+++ b/Documentation/features/io/dma-contiguous/arch-support.txt
@@ -6,7 +6,7 @@
     -----------------------
     |         arch |status|
     -----------------------
-    |       alpha: | TODO |
+    |       alpha: |  ok  |
     |         arc: | TODO |
     |         arm: |  ok  |
     |       arm64: |  ok  |
diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 6ccb2d230326..cddc1f3af209 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -30,6 +30,7 @@ config ALPHA
 	select HAVE_ARCH_COMPILER_H
 	select HAVE_ASM_MODVERSIONS
 	select HAVE_DEBUG_KMEMLEAK
+	select HAVE_DMA_CONTIGUOUS
 	select HAVE_GUP_FAST
 	select HAVE_KERNEL_BZIP2
 	select HAVE_KERNEL_GZIP
diff --git a/arch/alpha/kernel/pci_iommu.c b/arch/alpha/kernel/pci_iommu.c
index 955b6ca61627..0337bcef26cb 100644
--- a/arch/alpha/kernel/pci_iommu.c
+++ b/arch/alpha/kernel/pci_iommu.c
@@ -408,12 +408,17 @@ static void *alpha_pci_alloc_coherent(struct device *dev, size_t size,
 {
 	struct pci_dev *pdev = alpha_gendev_to_pci(dev);
 	void *cpu_addr;
-	long order = get_order(size);
+	struct page *page;
 
 	gfp &= ~GFP_DMA;
 
 try_again:
-	cpu_addr = (void *)__get_free_pages(gfp | __GFP_ZERO, order);
+	/* A low-memory retry must not return the same unsuitable CMA pages. */
+	page = (gfp & GFP_DMA) ? NULL : dma_alloc_contiguous(dev, size, gfp);
+	if (page)
+		cpu_addr = page_address(page);
+	else
+		cpu_addr = alloc_pages_exact(size, gfp | __GFP_ZERO);
 	if (! cpu_addr) {
 		printk(KERN_INFO "pci_alloc_consistent: "
 		       "get_free_pages failed from %ps\n",
@@ -426,7 +431,7 @@ static void *alpha_pci_alloc_coherent(struct device *dev, size_t size,
 
 	*dma_addrp = pci_map_single_1(pdev, virt_to_phys(cpu_addr), size, 0);
 	if (*dma_addrp == DMA_MAPPING_ERROR) {
-		free_pages((unsigned long)cpu_addr, order);
+		dma_free_contiguous(dev, virt_to_page(cpu_addr), size);
 		if (alpha_mv.mv_pci_tbi || (gfp & GFP_DMA))
 			return NULL;
 		/* The address doesn't fit required mask and we
@@ -453,7 +458,7 @@ static void alpha_pci_free_coherent(struct device *dev, size_t size,
 {
 	struct pci_dev *pdev = alpha_gendev_to_pci(dev);
 	dma_unmap_single(&pdev->dev, dma_addr, size, DMA_BIDIRECTIONAL);
-	free_pages((unsigned long)cpu_addr, get_order(size));
+	dma_free_contiguous(dev, virt_to_page(cpu_addr), size);
 
 	DBGA2("pci_free_consistent: [%llx,%zx] from %ps\n",
 	      dma_addr, size, __builtin_return_address(0));
diff --git a/arch/alpha/kernel/setup.c b/arch/alpha/kernel/setup.c
index c7b7ebcc47b6..0cd94c9023c1 100644
--- a/arch/alpha/kernel/setup.c
+++ b/arch/alpha/kernel/setup.c
@@ -30,6 +30,7 @@
 #include <linux/panic_notifier.h>
 #include <linux/platform_device.h>
 #include <linux/memblock.h>
+#include <linux/dma-map-ops.h>
 #include <linux/pci.h>
 #include <linux/seq_file.h>
 #include <linux/root_dev.h>
@@ -582,6 +583,20 @@ setup_arch(char **cmdline_p)
 	/* Parse early options before reserving memory for DMA CMA. */
 	parse_early_param();
 
+	/*
+	 * Prefer higher memory within the direct-map window, rather than
+	 * consuming scarce low memory with a bottom-up CMA reservation.
+	 * Explicit cma= ranges retain the generic command-line semantics.
+	 */
+	if (__direct_map_size) {
+		bool bottom_up = memblock_bottom_up();
+
+		memblock_set_bottom_up(false);
+		dma_contiguous_reserve(min_t(phys_addr_t, __direct_map_size,
+					     PFN_PHYS(max_low_pfn)));
+		memblock_set_bottom_up(bottom_up);
+	}
+
 	/* Reserve standard resources.  */
 	reserve_std_resources();
 
-- 
2.43.0