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

[Thomas Zimmermann]

Hi

Am 28.02.25 um 12:21 schrieb Jani Nikula:
> On Fri, 28 Feb 2025, Thomas Zimmermann <tzimmermann at suse.de> 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;
> The asymmetry in who gets and puts the ->dma_dev bugs me.
>
> You could avoid that by providing a helper for setting ->dma_dev, and
> having that get the device:
>
> int drm_dev_set_dma_device(struct drm_device *drm, struct device *dma_device)
> {
> 	dev->dma_dev = get_device(dma_device);
>
> 	return dev->dma_dev ? 0 : -ENODEV.
> }
>
> Taking drm/gm12u320 from patch 2 as an example, it would become:
>
> 	dma_device = usb_intf_get_dma_device(to_usb_interface(dev->dev))
> 	if (drm_dev_set_dma_device(drm, dma_device))
> 		drm_warn(dev, "buffer sharing not supported"); /* not an error */
> 	put_device(dma_device);
>
> Sligthly more verbose, but has a get/put pair in the driver, and a
> get/put pair in the core, instead of the asymmetry.
>
> I'm not insisting, but something to consider.

We're doing a get_device() on the regular parent kept in dev. So yeah, 
keeping a ref on the dma_dev would make sense.

Best regards
Thomas

>
>
> BR,
> Jani.
>
>
>>   	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;
>> +
>>   	/**
>>   	 * @managed:
>>   	 *
>> @@ -327,4 +344,24 @@ struct drm_device {
>>   	struct dentry *debugfs_root;
>>   };
>>   
>> +/**
>> + * drm_dev_dma_dev - returns the DMA device for a DRM device
>> + * @dev: DRM device
>> + *
>> + * Returns the DMA device of the given DRM device. This is usually
>> + * the same as the DRM device's parent. Devices on some peripheral
>> + * busses, such as USB, are incapable of performing DMA by themselves.
>> + * Drivers should set struct &drm_device.dma_dev to the bus controller
>> + * to make DMA work. Required for PRIME buffer import.
>> + *
>> + * Returns:
>> + * A DMA-capable device for the DRM device.
>> + */
>> +static inline struct device *drm_dev_dma_dev(struct drm_device *dev)
>> +{
>> +	if (dev->dma_dev)
>> +		return dev->dma_dev;
>> +	return dev->dev;
>> +}
>> +
>>   #endif

-- 
--
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)