Re: [RFC 1/2] virtio_console: Add support for DMA memory allocation

On Mon, Sep 03, 2012 at 04:57:45PM +0200, Sjur Brændeland wrote:

Hi Michael,

quoted

How does access to descriptors work in this setup?

When the ring is setup by remoteproc the descriptors are
also allocated using dma_alloc_coherent().

quoted

quoted

-static void free_buf(struct port_buffer *buf)
+/* Allcoate data buffer from DMA memory if requested */

typo

Thanks.

quoted

quoted

+static inline void *
+alloc_databuf(struct virtio_device *vdev, size_t size, dma_addr_t *dma_handle,
+                gfp_t flag)
 {
-     kfree(buf->buf);
+#ifdef CONFIG_HAS_DMA
+     if (virtio_has_feature(vdev, VIRTIO_CONSOLE_F_DMA_MEM)) {
+             struct device *dev = &vdev->dev;
+             /*
+              * Allocate DMA memory from ancestors. Finding the ancestor
+              * is a bit quirky when DMA_MEMORY_INCLUDES_CHILDREN is not
+              * implemented.
+              */
+             dev = dev->parent ? dev->parent : dev;
+             dev = dev->parent ? dev->parent : dev;
+             return dma_alloc_coherent(dev, size, dma_handle, flag);
+     }
+#endif

Are these ifdefs really needed? If DMA_MEM is set,
can't we use dma_alloc_coherent
unconditionally?

If an architecture do not support DMA you will get
a link error: "unknown symbol" for dma_alloc_coherent.

Regards,
Sjur

Yes, it even seems intentional.
But I have a question: can the device work without DMA?
Does not VIRTIO_CONSOLE_F_DMA_MEM mean dma api is required?
If yes you should just fail load.

Also let's add a wrapper at top of file so ifdefs
do not litter the code like this. For example:

#ifdef CONFIG_HAS_DMA
#define VIRTIO_CONSOLE_HAS_DMA (1)
#else
#define VIRTIO_CONSOLE_HAS_DMA (0)
#endif

Now use if instead of ifdef.

-- 
MST