Re: [Nouveau] [PATCH v4 2/6] drm/nouveau: map pages using DMA API on platform devices

From: Alexandre Courbot
Date: Thu Jul 10 2014 - 22:35:33 EST

On 07/10/2014 09:58 PM, Daniel Vetter wrote:
On Tue, Jul 08, 2014 at 05:25:57PM +0900, Alexandre Courbot wrote:
page_to_phys() is not the correct way to obtain the DMA address of a
buffer on a non-PCI system. Use the DMA API functions for this, which
are portable and will allow us to use other DMA API functions for
buffer synchronization.

Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
drivers/gpu/drm/nouveau/core/engine/device/base.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/core/engine/device/base.c b/drivers/gpu/drm/nouveau/core/engine/device/base.c
index 18c8c7245b73..e4e9e64988fe 100644
--- a/drivers/gpu/drm/nouveau/core/engine/device/base.c
+++ b/drivers/gpu/drm/nouveau/core/engine/device/base.c
@@ -489,7 +489,10 @@ nv_device_map_page(struct nouveau_device *device, struct page *page)
if (pci_dma_mapping_error(device->pdev, ret))
ret = 0;
} else {
- ret = page_to_phys(page);
+ ret = dma_map_page(&device->platformdev->dev, page, 0,
+ if (dma_mapping_error(&device->platformdev->dev, ret))
+ ret = 0;

return ret;
@@ -501,6 +504,9 @@ nv_device_unmap_page(struct nouveau_device *device, dma_addr_t addr)
if (nv_device_is_pci(device))
pci_unmap_page(device->pdev, addr, PAGE_SIZE,

pci_map/unmap alias to dma_unmap/map when called on the underlying struct
device embedded in pci_device (like for platform drivers). Dunno whether
it's worth to track a pointer to the struct device directly and always
call dma_unmap/map.

Isn't it (theoretically) possible to have a platform that does not use the DMA API for its PCI implementation and thus requires the pci_* functions to be called? I could not find such a case in -next, which suggests that all PCI platforms have been converted to the DMA API already and that we could indeed refactor this to always use the DMA functions.

But at the same time the way we use APIs should not be directed by their implementation, but by their intent - and unless the PCI API has been deprecated in some way (something I am not aware of), the rule is still that you should use it on a PCI device.

Just drive-by comment since I'm interested in how you solve this - i915
has similar fun with buffer sharing and coherent and non-coherent
platforms. Although we don't have fun with pci and non-pci based

Yeah, I am not familiar with i915 but it seems like we are on a similar boat here (excepted ARM is more constrained as to its memory mappings). The strategy in this series is, map buffers used by user-space cached and explicitly synchronize them (since the ownership transition from user to GPU is always clearly performed by syscalls), and use coherent mappings for buffers used by the kernel which are accessed more randomly. This has solved all our coherency issues and resulted in the best performance so far.

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