Re: [PATCH v4 04/33] drm/pagemap: Add DRM pagemap

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2025-01-29 at 11:51 -0800, Matthew Brost wrote:
> From: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> 
> Introduce drm_pagemap ops to map and unmap dma to VRAM resources. In
> the
> local memory case it's a matter of merely providing an offset into
> the
> device's physical address. For future p2p the map and unmap functions
> may
> encode as needed.
> 
> Similar to how dma-buf works, let the memory provider (drm_pagemap)
> provide
> the mapping functionality.

It should be noted that the long term idea for dma mapping is to have
that done by the client instead of by the memory provider, which Jason
reminded me of in a discussion on dri-devel. The dma-mapping here is
modeled after how it's done for dma-buf, where the exporter maps dma.

So following that, it might be that we should move these dma-mapping
ops to the drm_gpusvm().

The situation I can think of, where this might be a problem is that if
the device-private struct page to dma address mapping is not known to
the client.

/Thomas





> 
> v3:
>  - Move to drm level include
> v4:
>  - Fix kernel doc (G.G.)
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> ---
>  include/drm/drm_pagemap.h | 105
> ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 105 insertions(+)
>  create mode 100644 include/drm/drm_pagemap.h
> 
> diff --git a/include/drm/drm_pagemap.h b/include/drm/drm_pagemap.h
> new file mode 100644
> index 000000000000..2b610ccf7e30
> --- /dev/null
> +++ b/include/drm/drm_pagemap.h
> @@ -0,0 +1,105 @@
> +/* SPDX-License-Identifier: MIT */
> +#ifndef _DRM_PAGEMAP_H_
> +#define _DRM_PAGEMAP_H_
> +
> +#include <linux/dma-direction.h>
> +#include <linux/hmm.h>
> +#include <linux/types.h>
> +
> +struct drm_pagemap;
> +struct device;
> +
> +/**
> + * enum drm_interconnect_protocol - Used to identify an interconnect
> protocol.
> + */
> +enum drm_interconnect_protocol {
> +	DRM_INTERCONNECT_SYSTEM,    /* DMA map is system pages. */
> +	DRM_INTERCONNECT_PCIE_P2P,  /* DMA map is PCIE P2P */
> +	DRM_INTERCONNECT_DRIVER,    /* DMA map is driver defined */
> +	/* A driver can add private values beyond
> DRM_INTERCONNECT_DRIVER */
> +};
> +
> +/**
> + * struct drm_pagemap_dma_addr - DMA address representation.
> + * @addr: The dma address or driver-defined address for driver
> private interconnects.
> + * @proto: The interconnect protocol.
> + * @order: The page order of the dma mapping. (Size is PAGE_SIZE <<
> order).
> + * @dir: The DMA direction.
> + *
> + * Note: There is room for improvement here. We should be able to
> pack into
> + * 64 bits.
> + */
> +struct drm_pagemap_dma_addr {
> +	dma_addr_t addr;
> +	u64 proto : 54;
> +	u64 order : 8;
> +	u64 dir : 2;
> +};
> +
> +/**
> + * drm_pagemap_dma_addr_encode() - Encode a dma address with
> metadata
> + * @addr: The dma address or driver-defined address for driver
> private interconnects.
> + * @proto: The interconnect protocol.
> + * @order: The page order of the dma mapping. (Size is PAGE_SIZE <<
> order).
> + * @dir: The DMA direction.
> + *
> + * Return: A struct drm_pagemap_dma_addr encoding the above
> information.
> + */
> +static inline struct drm_pagemap_dma_addr
> +drm_pagemap_dma_addr_encode(dma_addr_t addr,
> +			    enum drm_interconnect_protocol proto,
> +			    unsigned int order,
> +			    enum dma_data_direction dir)
> +{
> +	return (struct drm_pagemap_dma_addr) {
> +		.addr = addr,
> +		.proto = proto,
> +		.order = order,
> +		.dir = dir,
> +	};
> +}
> +
> +/**
> + * struct drm_pagemap_ops: Ops for a drm-pagemap.
> + */
> +struct drm_pagemap_ops {
> +	/**
> +	 * @map_dma: Map for dma access or provide a virtual address
> suitable for
> +	 *
> +	 * @dpagemap: The struct drm_pagemap for the page.
> +	 * @dev: The dma mapper.
> +	 * @page: The page to map.
> +	 * @order: The page order of the dma mapping. (Size is
> PAGE_SIZE << order).
> +	 * @dir: The transfer direction.
> +	 */
> +	struct drm_pagemap_dma_addr (*map_dma)(struct drm_pagemap
> *dpagemap,
> +					       struct device *dev,
> +					       struct page *page,
> +					       unsigned int order,
> +					       enum
> dma_data_direction dir);
> +
> +	/**
> +	 * @unmap_dma: Unmap a dma address previously obtained using
> @map_dma.
> +	 *
> +	 * @dpagemap: The struct drm_pagemap for the mapping.
> +	 * @dev: The dma unmapper.
> +	 * @addr: The dma address obtained when mapping.
> +	 */
> +	void (*unmap_dma)(struct drm_pagemap *dpagemap,
> +			  struct device *dev,
> +			  struct drm_pagemap_dma_addr addr);
> +
> +};
> +
> +/**
> + * struct drm_pagemap: Additional information for a struct
> dev_pagemap
> + * used for device p2p handshaking.
> + * @ops: The struct drm_pagemap_ops.
> + * @dev: The struct drevice owning the device-private memory.
> + */
> +struct drm_pagemap {
> +	const struct drm_pagemap_ops *ops;
> +	struct device *dev;
> +};
> +
> +#endif