[RFC v2 PATCH 1/3] ARM: NOMMU: introduce dma operations for noMMU

[Vladimir Murzin]

Hello Benjamin,

On 02/01/17 15:26, Benjamin Gaignard wrote:
> Hello Vladimir,
> 
> I have tested your patch on my setup (stm32f4: no MMU, no MPU) where
> I'm writing display driver.
> This driver use dma_alloc_wc() and dma_mmap_wc() for frame buffer
> allocation and mmapping.
> 
> In dma-mapping-nommu.c you haven't implement dma_map_ops.mmap so
> obviously my driver
> doesn't work with your code.
> In current implementation it is buggy too but I submit a patch to fix
> that problem:
> http://www.armlinux.org.uk/developer/patches/viewpatch.php?id=8633/1
> 
> Could it be possible for you to include mmap support in dma-mapping-nommu.c ?
> 

IIRC, stm32f4 is Cortex-M4, so no cache support and it means that it uses
dma_noop_ops. I offloaded mmap to common implementation, but completely forgot
it has the same restriction as arm counterpart. Regardless, thanks for
noticing that!

It seems that I need to check that mapping is done from DMA coherent area (I'm
moving to dma-coherent and here we have dma_mmap_from_coherent for that) and
something like bellow for dma_noop_ops:

diff --git a/lib/dma-noop.c b/lib/dma-noop.c
index 3d766e7..d838b88 100644
--- a/lib/dma-noop.c
+++ b/lib/dma-noop.c
@@ -64,6 +64,25 @@ static int dma_noop_supported(struct device *dev, u64 mask)
 	return 1;
 }
 
+static int dma_noop_mmap(struct device *dev, struct vm_area_struct *vma,
+			 void *cpu_addr, dma_addr_t dma_addr, size_t size)
+{
+	unsigned long user_count = vma_pages(vma);
+	unsigned long count = PAGE_ALIGN(size) >> PAGE_SHIFT;
+	unsigned long pfn = page_to_pfn(virt_to_page(cpu_addr));
+	unsigned long off = vma->vm_pgoff;
+	int ret = -ENXIO;
+
+	if (off < count && user_count <= (count - off)) {
+		ret = remap_pfn_range(vma, vma->vm_start,
+				      pfn + off,
+				      user_count << PAGE_SHIFT,
+				      vma->vm_page_prot);
+	}
+
+	return ret;
+}
+
 struct dma_map_ops dma_noop_ops = {
 	.alloc			= dma_noop_alloc,
 	.free			= dma_noop_free,
@@ -71,6 +90,7 @@ struct dma_map_ops dma_noop_ops = {
 	.map_sg			= dma_noop_map_sg,
 	.mapping_error		= dma_noop_mapping_error,
 	.dma_supported		= dma_noop_supported,
+	.mmap			= dma_noop_mmap,
 };
 
 EXPORT_SYMBOL(dma_noop_ops);

I'd be glad to hear if it works for you.

Cheers
Vladimir

> Regards,
> Benjamin