Re: [Nouveau] [PATCH v4 4/6] drm/nouveau: synchronize BOs when required

From: Alexandre Courbot
Date: Thu Jul 10 2014 - 22:40:38 EST

On 07/10/2014 10:04 PM, Daniel Vetter wrote:
On Tue, Jul 08, 2014 at 05:25:59PM +0900, Alexandre Courbot wrote:
On architectures for which access to GPU memory is non-coherent,
caches need to be flushed and invalidated explicitly when BO control
changes between CPU and GPU.

This patch adds buffer synchronization functions which invokes the
correct API (PCI or DMA) to ensure synchronization is effective.

Based on the TTM DMA cache helper patches by Lucas Stach.

Signed-off-by: Lucas Stach <dev@xxxxxxxxxx>
Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
drivers/gpu/drm/nouveau/nouveau_bo.c | 56 +++++++++++++++++++++++++++++++++++
drivers/gpu/drm/nouveau/nouveau_bo.h | 2 ++
drivers/gpu/drm/nouveau/nouveau_gem.c | 12 ++++++++
3 files changed, 70 insertions(+)

diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c
index 67e9e8e2e2ec..47e4e8886769 100644
--- a/drivers/gpu/drm/nouveau/nouveau_bo.c
+++ b/drivers/gpu/drm/nouveau/nouveau_bo.c
@@ -402,6 +402,60 @@ nouveau_bo_unmap(struct nouveau_bo *nvbo)

+nouveau_bo_sync_for_device(struct nouveau_bo *nvbo)
+ struct nouveau_drm *drm = nouveau_bdev(nvbo->bo.bdev);
+ struct nouveau_device *device = nouveau_dev(drm->dev);
+ struct ttm_dma_tt *ttm_dma = (struct ttm_dma_tt *)nvbo->bo.ttm;
+ int i;
+ if (!ttm_dma)
+ return;
+ if (nv_device_is_cpu_coherent(device) || nvbo->force_coherent)
+ return;

Is the is_cpu_coherent check really required? On coherent platforms the
sync_for_foo should be a noop. It's the dma api's job to encapsulate this
knowledge so that drivers can be blissfully ignorant. The explicit
is_coherent check makes this a bit leaky. And same comment that underlying
the bus-specifics dma-mapping functions are identical.

I think you are right, the is_cpu_coherent check should not be needed here. I still think we should have separate paths for the PCI/DMA cases though, unless you can point me to a source that clearly states that the PCI API is deprecated and that DMA should be used instead.
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