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

[Ben Skeggs]

On Fri, Jul 11, 2014 at 12:35 PM, Alexandre Courbot <acourbot@xxxxxxxxxx> wrote:
> 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,
>>> +                                  PAGE_SIZE, DMA_BIDIRECTIONAL);
>>> +               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_DMA_BIDIRECTIONAL);
>>
>>
>> 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
>> platforms.
>
>
> 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.
I wonder if we might want to use unsnooped cached mappings of pages on
non-ARM platforms also, to avoid the overhead of the cache snooping?

>
>
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