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

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Date:	Fri, 11 Jul 2014 11:40:27 +0900
From:	Alexandre Courbot <acourbot@...dia.com>
To:	Daniel Vetter <daniel@...ll.ch>
CC:	Ben Skeggs <bskeggs@...hat.com>, David Airlie <airlied@...ux.ie>,
	David Herrmann <dh.herrmann@...il.com>,
	Lucas Stach <dev@...xeye.de>,
	Thierry Reding <thierry.reding@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	<nouveau@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
	<dri-devel@...ts.freedesktop.org>, <linux-tegra@...r.kernel.org>,
	Alexandre Courbot <gnurou@...il.com>
Subject: Re: [Nouveau] [PATCH v4 4/6] drm/nouveau: synchronize BOs when required

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@...xeye.de>
>> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
>> ---
>>   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)
>>   		ttm_bo_kunmap(&nvbo->kmap);
>>   }
>>
>> +void
>> +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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