[PATCH 1/4] drm/prime: Support dedicated DMA device for dma-buf imports

[Thomas Zimmermann]

Hi

Am 03.03.25 um 10:57 schrieb Maxime Ripard:
> Hi Thomas,
>
> On Fri, Feb 28, 2025 at 10:32:51AM +0100, Thomas Zimmermann wrote:
>> Importing dma-bufs via PRIME requires a DMA-capable device. Devices on
>> peripheral busses, such as USB, often cannot perform DMA by themselves.
>> Without DMA-capable device PRIME import fails. DRM drivers for USB
>> devices already use a separate DMA device for dma-buf imports. Make the
>> mechanism generally available.
>>
>> Add the field dma_dev to struct drm_device to refer to the device's DMA
>> device. For USB this should be the USB controller. Use dma_dev in the
>> PRIME import helpers, if set.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
>> ---
>>   drivers/gpu/drm/drm_drv.c   |  2 ++
>>   drivers/gpu/drm/drm_prime.c |  2 +-
>>   include/drm/drm_device.h    | 37 +++++++++++++++++++++++++++++++++++++
>>   3 files changed, 40 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>> index 17fc5dc708f4..f8c3c9f77d22 100644
>> --- a/drivers/gpu/drm/drm_drv.c
>> +++ b/drivers/gpu/drm/drm_drv.c
>> @@ -654,6 +654,8 @@ static void drm_dev_init_release(struct drm_device *dev, void *res)
>>   {
>>   	drm_fs_inode_free(dev->anon_inode);
>>   
>> +	put_device(dev->dma_dev);
>> +	dev->dma_dev = NULL;
>>   	put_device(dev->dev);
>>   	/* Prevent use-after-free in drm_managed_release when debugging is
>>   	 * enabled. Slightly awkward, but can't really be helped. */
>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>> index 32a8781cfd67..258858f2f8dd 100644
>> --- a/drivers/gpu/drm/drm_prime.c
>> +++ b/drivers/gpu/drm/drm_prime.c
>> @@ -1004,7 +1004,7 @@ EXPORT_SYMBOL(drm_gem_prime_import_dev);
>>   struct drm_gem_object *drm_gem_prime_import(struct drm_device *dev,
>>   					    struct dma_buf *dma_buf)
>>   {
>> -	return drm_gem_prime_import_dev(dev, dma_buf, dev->dev);
>> +	return drm_gem_prime_import_dev(dev, dma_buf, drm_dev_dma_dev(dev));
>>   }
>>   EXPORT_SYMBOL(drm_gem_prime_import);
>>   
>> diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
>> index 6ea54a578cda..a24cac4b2077 100644
>> --- a/include/drm/drm_device.h
>> +++ b/include/drm/drm_device.h
>> @@ -64,6 +64,23 @@ struct drm_device {
>>   	/** @dev: Device structure of bus-device */
>>   	struct device *dev;
>>   
>> +	/**
>> +	 * @dma_dev:
>> +	 *
>> +	 * Device for DMA operations. Only required if the device @dev
>> +	 * cannot perform DMA by itself. Should be NULL otherwise.
>> +	 *
>> +	 * Devices on USB and other peripheral busses cannot perform DMA
>> +	 * by themselves. The @dma_dev field should point the bus controller
>> +	 * that does DMA on behalve of such a device. Required for importing
>> +	 * buffers via dma-buf.
>> +	 *
>> +	 * If set, the DRM driver has to acquire a reference on the DMA
>> +	 * device, which will be owned and released automatically by the
>> +	 * DRM core.
>> +	 */
>> +	struct device *dma_dev;
>> +
> It looks good to me in general, but this is also useful with ARM
> platform is general. On those, the DRM device is bound to a virtual
> device (and thus can't do DMA), but the HW accesses will be done by
> one or more HW controllers that are part of the overall DRM driver.
>
> Thus, we typically have to make hacks to copy to the virtual device DMA
> setup from the actual device doing the DMA accesses.
>
> See for example
> https://elixir.bootlin.com/linux/v6.13.5/source/drivers/gpu/drm/vc4/vc4_drv.c#L313
> https://elixir.bootlin.com/linux/v6.13.5/source/drivers/gpu/drm/sun4i/sun4i_backend.c#L797
>
> It's probably worth documenting.

Sure, I can do that.

In the series' cover letter, I mention that the GEM-DMA code could use 
the dma_dev. It seems that these drivers would benefit from such a change.

Best regards
Thomas

>
> Maxime

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)