Re: [PATCH 6/6] virtio: add api virtio_dma_map() for advance dma

On Mon, 10 Jan 2022 02:12:23 -0500, Michael S. Tsirkin [off-list ref] wrote:

On Fri, Jan 07, 2022 at 02:33:06PM +0800, Xuan Zhuo wrote:

quoted

Added virtio_dma_map() to map DMA addresses for virtual memory in
advance. The purpose of adding this function is to check
vring_use_dma_api() for virtio dma operation and get vdev->dev.parent as
the parameter of dma_map_page().

Added virtio_dma_unmap() for unmap DMA address.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

OK but where are the users for this new API?

My subsequent patch set (virtio net supports AF_XDP) will use this API.

quoted

---
 drivers/virtio/virtio_ring.c | 47 ++++++++++++++++++++++++++++++++++++
 include/linux/virtio.h       |  9 +++++++
 2 files changed, 56 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index e165bc2e1344..f4a0fb85df27 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -2472,4 +2472,51 @@ const struct vring *virtqueue_get_vring(struct virtqueue *vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_vring);

+/**
+ * virtio_dma_map - get the DMA addr of the memory for virtio device
+ * @vdev: virtio device
+ * @page: the page of the memory to DMA
+ * @offset: the offset of the memory inside page
+ * @length: memory length
+ * @dir: DMA direction
+ *
+ * Returns the DMA addr. Zero means error.

Should not drivers use a variant of dma_mapping_error to check?

The purpose of this is to wrap vdev->dev.parent. Otherwise, the driver will
directly access vdev->dev.parent.

quoted

+ */
+dma_addr_t virtio_dma_map(struct virtio_device *vdev,
+			  struct page *page, size_t offset,
+			  unsigned int length,
+			  enum dma_data_direction dir)
+{
+	dma_addr_t addr;
+
+	if (!vring_use_dma_api(vdev))
+		return page_to_phys(page) + offset;
+
+	addr = dma_map_page(vdev->dev.parent, page, offset, length, dir);
+
+	if (dma_mapping_error(vdev->dev.parent, addr))
+		return 0;
+
+	return addr;
+}
+EXPORT_SYMBOL_GPL(virtio_dma_map);

Yes it's 0, but you should really use DMA_MAPPING_ERROR.

Yes, I should use DMA_MAPPING_ERROR.

Thanks.

quoted

+
+/**
+ * virtio_dma_unmap - unmap DMA addr
+ * @vdev: virtio device
+ * @dma: DMA address
+ * @length: memory length
+ * @dir: DMA direction
+ */
+void virtio_dma_unmap(struct virtio_device *vdev,
+		      dma_addr_t dma, unsigned int length,
+		      enum dma_data_direction dir)
+{
+	if (!vring_use_dma_api(vdev))
+		return;
+
+	dma_unmap_page(vdev->dev.parent, dma, length, dir);
+}
+EXPORT_SYMBOL_GPL(virtio_dma_unmap);
+
 MODULE_LICENSE("GPL");

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 41edbc01ffa4..6e6c6e18ecf8 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -9,6 +9,7 @@
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
 #include <linux/gfp.h>
+#include <linux/dma-mapping.h>

 /**
  * virtqueue - a queue to register buffers for sending or receiving.

@@ -195,4 +196,12 @@ void unregister_virtio_driver(struct virtio_driver *drv);
 #define module_virtio_driver(__virtio_driver) \
 	module_driver(__virtio_driver, register_virtio_driver, \
 			unregister_virtio_driver)
+
+dma_addr_t virtio_dma_map(struct virtio_device *vdev,
+			  struct page *page, size_t offset,
+			  unsigned int length,
+			  enum dma_data_direction dir);
+void virtio_dma_unmap(struct virtio_device *vdev,
+		      dma_addr_t dma, unsigned int length,
+		      enum dma_data_direction dir);
 #endif /* _LINUX_VIRTIO_H */
--

2.31.0

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