Re: [Xen-devel] [PATCH v2 5/9] xen/gntdev: Allow mappings for DMA buffers

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On 06/04/2018 11:12 PM, Boris Ostrovsky wrote:
On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>

Allow mappings for DMA backed  buffers if grant table module
supports such: this extends grant device to not only map buffers
made of balloon pages, but also from buffers allocated with
dma_alloc_xxx.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
---
  drivers/xen/gntdev.c      | 99 ++++++++++++++++++++++++++++++++++++++-
  include/uapi/xen/gntdev.h | 15 ++++++
  2 files changed, 112 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index bd56653b9bbc..9813fc440c70 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -37,6 +37,9 @@
  #include <linux/slab.h>
  #include <linux/highmem.h>
  #include <linux/refcount.h>
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+#include <linux/of_device.h>
+#endif
#include <xen/xen.h>
  #include <xen/grant_table.h>
@@ -72,6 +75,11 @@ struct gntdev_priv {
  	struct mutex lock;
  	struct mm_struct *mm;
  	struct mmu_notifier mn;
+
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	/* Device for which DMA memory is allocated. */
+	struct device *dma_dev;
+#endif
  };
struct unmap_notify {
@@ -96,10 +104,27 @@ struct grant_map {
  	struct gnttab_unmap_grant_ref *kunmap_ops;
  	struct page **pages;
  	unsigned long pages_vm_start;
+
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	/*
+	 * If dmabuf_vaddr is not NULL then this mapping is backed by DMA
+	 * capable memory.
+	 */
+
+	struct device *dma_dev;
+	/* Flags used to create this DMA buffer: GNTDEV_DMA_FLAG_XXX. */
+	int dma_flags;
+	void *dma_vaddr;
+	dma_addr_t dma_bus_addr;
+	/* This is required for gnttab_dma_{alloc|free}_pages. */
How about

/* Needed to avoid allocation in gnttab_dma_free_pages(). */

Ok
+	xen_pfn_t *frames;
+#endif
  };
static int unmap_grant_pages(struct grant_map *map, int offset, int pages); +static struct miscdevice gntdev_miscdev;
+
  /* ------------------------------------------------------------------ */
static void gntdev_print_maps(struct gntdev_priv *priv,
@@ -121,8 +146,27 @@ static void gntdev_free_map(struct grant_map *map)
  	if (map == NULL)
  		return;
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	if (map->dma_vaddr) {
+		struct gnttab_dma_alloc_args args;
+
+		args.dev = map->dma_dev;
+		args.coherent = map->dma_flags & GNTDEV_DMA_FLAG_COHERENT;
+		args.nr_pages = map->count;
+		args.pages = map->pages;
+		args.frames = map->frames;
+		args.vaddr = map->dma_vaddr;
+		args.dev_bus_addr = map->dma_bus_addr;
+
+		gnttab_dma_free_pages(&args);
+	} else
+#endif
  	if (map->pages)
  		gnttab_free_pages(map->count, map->pages);
+
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	kfree(map->frames);
+#endif

Can this be done under if (map->dma_vaddr) ?

  In other words, is it
possible for dma_vaddr to be NULL and still have unallocated frames pointer?
It is possible to have vaddr == NULL and frames != NULL as we
allocate frames outside of gnttab_dma_alloc_pages which
may fail. Calling kfree on NULL pointer is safe, so
I see no reason to change this code.

  	kfree(map->pages);
  	kfree(map->grants);
  	kfree(map->map_ops);
@@ -132,7 +176,8 @@ static void gntdev_free_map(struct grant_map *map)
  	kfree(map);
  }
-static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
+static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count,
+					  int dma_flags)
  {
  	struct grant_map *add;
  	int i;
@@ -155,6 +200,37 @@ static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
  	    NULL == add->pages)
  		goto err;
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	add->dma_flags = dma_flags;
+
+	/*
+	 * Check if this mapping is requested to be backed
+	 * by a DMA buffer.
+	 */
+	if (dma_flags & (GNTDEV_DMA_FLAG_WC | GNTDEV_DMA_FLAG_COHERENT)) {
+		struct gnttab_dma_alloc_args args;
+
+		add->frames = kcalloc(count, sizeof(add->frames[0]),
+				      GFP_KERNEL);
+		if (!add->frames)
+			goto err;
+
+		/* Remember the device, so we can free DMA memory. */
+		add->dma_dev = priv->dma_dev;
+
+		args.dev = priv->dma_dev;
+		args.coherent = dma_flags & GNTDEV_DMA_FLAG_COHERENT;
+		args.nr_pages = count;
+		args.pages = add->pages;
+		args.frames = add->frames;
+
+		if (gnttab_dma_alloc_pages(&args))
+			goto err;
+
+		add->dma_vaddr = args.vaddr;
+		add->dma_bus_addr = args.dev_bus_addr;
+	} else
+#endif
  	if (gnttab_alloc_pages(count, add->pages))
  		goto err;
@@ -325,6 +401,14 @@ static int map_grant_pages(struct grant_map *map)
  		map->unmap_ops[i].handle = map->map_ops[i].handle;
  		if (use_ptemod)
  			map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+		else if (map->dma_vaddr) {
+			unsigned long mfn;
+
+			mfn = __pfn_to_mfn(page_to_pfn(map->pages[i]));

Not pfn_to_mfn()?
I'd love to, but pfn_to_mfn is only defined for x86, not ARM: [1] and [2]
Thus,

drivers/xen/gntdev.c:408:10: error: implicit declaration of function ‘pfn_to_mfn’ [-Werror=implicit-function-declaration]
    mfn = pfn_to_mfn(page_to_pfn(map->pages[i]));

So, I'll keep __pfn_to_mfn


-boris
Thank you,
Oleksandr
+			map->unmap_ops[i].dev_bus_addr = __pfn_to_phys(mfn);
+		}
+#endif
  	}
  	return err;
  }
@@ -548,6 +632,17 @@ static int gntdev_open(struct inode *inode, struct file *flip)
  	}
flip->private_data = priv;
+#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
+	priv->dma_dev = gntdev_miscdev.this_device;
+
+	/*
+	 * The device is not spawn from a device tree, so arch_setup_dma_ops
+	 * is not called, thus leaving the device with dummy DMA ops.
+	 * Fix this call of_dma_configure() with a NULL node to set
+	 * default DMA ops.
+	 */
+	of_dma_configure(priv->dma_dev, NULL);
+#endif
  	pr_debug("priv %p\n", priv);
return 0;
@@ -589,7 +684,7 @@ static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv,
  		return -EINVAL;
err = -ENOMEM;
-	map = gntdev_alloc_map(priv, op.count);
+	map = gntdev_alloc_map(priv, op.count, 0 /* This is not a dma-buf. */);
  	if (!map)
  		return err;
diff --git a/include/uapi/xen/gntdev.h b/include/uapi/xen/gntdev.h
index 6d1163456c03..4b9d498a31d4 100644
--- a/include/uapi/xen/gntdev.h
+++ b/include/uapi/xen/gntdev.h
@@ -200,4 +200,19 @@ struct ioctl_gntdev_grant_copy {
  /* Send an interrupt on the indicated event channel */
  #define UNMAP_NOTIFY_SEND_EVENT 0x2
+/*
+ * Flags to be used while requesting memory mapping's backing storage
+ * to be allocated with DMA API.
+ */
+
+/*
+ * The buffer is backed with memory allocated with dma_alloc_wc.
+ */
+#define GNTDEV_DMA_FLAG_WC		(1 << 0)
+
+/*
+ * The buffer is backed with memory allocated with dma_alloc_coherent.
+ */
+#define GNTDEV_DMA_FLAG_COHERENT	(1 << 1)
+
  #endif /* __LINUX_PUBLIC_GNTDEV_H__ */

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[1] https://elixir.bootlin.com/linux/v4.17/ident/pfn_to_mfn
[2] https://elixir.bootlin.com/linux/v4.17/ident/__pfn_to_mfn
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