[RFC PATCH v2] Utilize the PCI API in the TTM framework.

Thomas Hellstrom thomas at shipmail.org
Sat Jan 8 02:41:50 PST 2011

Hi, Konrad!

Just back from vacation. I'll try to review this patch set in the 
upcoming week!


On 01/07/2011 06:11 PM, Konrad Rzeszutek Wilk wrote:
> Attached is a set of patches that make it possible for drivers using TTM API
> (nouveau and radeon graphic drivers) to work under Xen. The explanation
> is a bit complex and I am not sure if I am explaining it that well..so if
> something is unclear please do ping me.
> Changes since v1: [https://lkml.org/lkml/2010/12/6/516]
>   - Cleaned up commit message (forgot to add my SoB).
> Short explanation of problem: What we are hitting under Xen is that instead
> of programming the GART with the physical DMA address of the TTM page, we
> end up programming the bounce buffer DMA (bus) address!
> Long explanation:
> The reason we end up doing this is that:
>   1). alloc_page with GFP_DMA32 does not allocate "real" (Machine Frame Numbers
>       - MFN) under the 4GB in Xen. That is b/c if actually made the pages underneath
>       4GB available to the the Linux page allocator we would not be able to
>       give those to other guest devices. This would mean if we tried to pass
>       in a USB device to one guest and in another were running the Xorg server
>       we wouldn't be able to do so as we would run out of pages under 4GB. So
>       pages that we get from alloc_page have a PFN that is under 4GB but in
>       reality the real physical address (MFN) is above 4GB. Ugh..
>   2). The backends for "struct ttm_backend_func" utilize the PCI API. When
>       they get a page allocated via alloc_page, the use 'pci_map_page' and
>       program the DMA (bus) address in the GART - which is correct. But then
>       the calls that kick off the graphic driver to process the pages do not
>       use the pci_page_sync_* calls. If the physical address of the page
>       is the same as the DMA bus address returned from pci_map_page then there
>       are no trouble. But if they are different:
> 	virt_to_phys(page_address(p)) != pci_map_page(p,..)
>       then the graphic card fetches data from the DMA (bus) address (so the
>       value returned from pci_map_page). The data however that the user wrote
>       to (the page p) ends up being untouched. You are probably saying:
>       "What? Nonsense, we stitch the page in ttm_bo_vm_fault using the PFN
>       and .. and even if the virt_to_phys(page_address(p)) != pci_map_page(p)
>       the GART ends up with the bus (DMA) address of the PFN!" That is true.
>       But if you combine this with 1) where you end up with page that is
>       above the dma_mask (even if you called it with GFP_DMA32) and then
>       make a call on pci_map_page you would end up with a bounce buffer!
> The problem above can be easily reproduced on bare-metal if you pass in
> "swiotlb=force iommu=soft".
> There are two ways of fixing this:
>   1). Use the 'dma_alloc_coherent' (or pci_alloc_consistent if there is
>       struct pcidev present), instead of alloc_page for GFP_DMA32. The
>       'dma_alloc_coherent' guarantees that the allocated page fits
>       within the device dma_mask (or uses the default DMA32 if no device
>       is passed in). This also guarantees that any subsequent call
>       to the PCI API for this page will return the same DMA (bus) address
>       as the first call (so pci_alloc_consistent, and then pci_map_page
>       will give the same DMA bus address).
>   2). Use the pci_sync_range_* after sending a page to the graphics
>       engine. If the bounce buffer is used then we end up copying the
>       pages.
>   3). This one I really don't want to think about, but I should mention
>       it. Alter the alloc_page and its backend to know about Xen.
>       The pushback from the MM guys will be probably: "why not use the PCI API.."
> So attached is a draft set of patches that use solution #1. I've tested
> this on baremetal (and Xen) on PCIe Radeon and nouveau cards with success.
> 1) The 'NULL' when doing dma_alloc_coherent is unsightly. I was toying
> with modifying the TTM API to pass in 'struct device' or 'struct pci_device'
> but figured it would make first sense to get your guys input before heading that route.
> This patch-set is also available at:
> git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git devel/ttm.pci-api.v3
> Konrad Rzeszutek Wilk (5):
>        ttm: Introduce a placeholder for DMA (bus) addresses.
>        tm: Utilize the dma_addr_t array for pages that are to in DMA32 pool.
>        ttm: Expand (*populate) to support an array of DMA addresses.
>        radeon/ttm/PCIe: Use dma_addr if TTM has set it.
>        nouveau/ttm/PCIe: Use dma_addr if TTM has set it.
> And diffstat:
>   drivers/gpu/drm/nouveau/nouveau_sgdma.c |   31 +++++++++++++++++++-------
>   drivers/gpu/drm/radeon/radeon.h         |    4 ++-
>   drivers/gpu/drm/radeon/radeon_gart.c    |   36 ++++++++++++++++++++++--------
>   drivers/gpu/drm/radeon/radeon_ttm.c     |    8 +++++-
>   drivers/gpu/drm/ttm/ttm_agp_backend.c   |    3 +-
>   drivers/gpu/drm/ttm/ttm_page_alloc.c    |   35 +++++++++++++++++++++++++-----
>   drivers/gpu/drm/ttm/ttm_tt.c            |   12 +++++++--
>   drivers/gpu/drm/vmwgfx/vmwgfx_buffer.c  |    3 +-
>   include/drm/ttm/ttm_bo_driver.h         |    6 ++++-
>   include/drm/ttm/ttm_page_alloc.h        |    8 +++++-
>   10 files changed, 111 insertions(+), 35 deletions(-)

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