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

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Date:	Fri, 11 Jul 2014 11:35:23 +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 2/6] drm/nouveau: map pages using DMA API
 on platform devices

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@...dia.com>
>> ---
>>   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.

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