Re: [Intel-gfx] [PATCH v2 1/3] dma-buf: Add struct dma-buf-map for storing struct dma_buf.vaddr_ptr

["Christian König" <christian.koenig@amd.com>]

Am 23.09.20 um 14:32 schrieb Thomas Zimmermann:
> The new type struct dma_buf_map represents a mapping of dma-buf memory
> into kernel space. It contains a flag, is_iomem, that signals users to
> access the mapped memory with I/O operations instead of regular loads
> and stores.
>
> It was assumed that DMA buffer memory can be accessed with regular load
> and store operations. Some architectures, such as sparc64, require the
> use of I/O operations to access dma-map buffers that are located in I/O
> memory. Providing struct dma_buf_map allows drivers to implement this.
> This was specifically a problem when refreshing the graphics framebuffer
> on such systems. [1]
>
> As the first step, struct dma_buf stores an instance of struct dma_buf_map
> internally. Afterwards, dma-buf's vmap and vunmap interfaces are be
> converted. Finally, affected drivers can be fixed.
>
> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fdri-devel%2F20200725191012.GA434957%40ravnborg.org%2F&data=02%7C01%7Cchristian.koenig%40amd.com%7C54486b9682654f3950b808d85fbcb1d3%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637364611338153209&sdata=%2BZm7t8OcgkIxnY%2FdZSLhSbKC7t1y4VW5lINFKwCQv3A%3D&reserved=0

Only two nit picks below, apart from that Reviewed-by: Christian König 
<christian.koenig@amd.com>

>
> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
> ---
>   Documentation/driver-api/dma-buf.rst |  3 +
>   drivers/dma-buf/dma-buf.c            | 14 ++---
>   include/linux/dma-buf-map.h          | 87 ++++++++++++++++++++++++++++
>   include/linux/dma-buf.h              |  3 +-
>   4 files changed, 99 insertions(+), 8 deletions(-)
>   create mode 100644 include/linux/dma-buf-map.h
>
> diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
> index 13ea0cc0a3fa..3244c600a9a1 100644
> --- a/Documentation/driver-api/dma-buf.rst
> +++ b/Documentation/driver-api/dma-buf.rst
> @@ -115,6 +115,9 @@ Kernel Functions and Structures Reference
>   .. kernel-doc:: include/linux/dma-buf.h
>      :internal:
>   
> +.. kernel-doc:: include/linux/dma-buf-map.h
> +   :internal:
> +
>   Reservation Objects
>   -------------------
>   
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 58564d82a3a2..5e849ca241a0 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1207,12 +1207,12 @@ void *dma_buf_vmap(struct dma_buf *dmabuf)
>   	mutex_lock(&dmabuf->lock);
>   	if (dmabuf->vmapping_counter) {
>   		dmabuf->vmapping_counter++;
> -		BUG_ON(!dmabuf->vmap_ptr);
> -		ptr = dmabuf->vmap_ptr;
> +		BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
> +		ptr = dmabuf->vmap_ptr.vaddr;
>   		goto out_unlock;
>   	}
>   
> -	BUG_ON(dmabuf->vmap_ptr);
> +	BUG_ON(dma_buf_map_is_set(&dmabuf->vmap_ptr));
>   
>   	ptr = dmabuf->ops->vmap(dmabuf);
>   	if (WARN_ON_ONCE(IS_ERR(ptr)))
> @@ -1220,7 +1220,7 @@ void *dma_buf_vmap(struct dma_buf *dmabuf)
>   	if (!ptr)
>   		goto out_unlock;
>   
> -	dmabuf->vmap_ptr = ptr;
> +	dmabuf->vmap_ptr.vaddr = ptr;
>   	dmabuf->vmapping_counter = 1;
>   
>   out_unlock:
> @@ -1239,15 +1239,15 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
>   	if (WARN_ON(!dmabuf))
>   		return;
>   
> -	BUG_ON(!dmabuf->vmap_ptr);
> +	BUG_ON(dma_buf_map_is_null(&dmabuf->vmap_ptr));
>   	BUG_ON(dmabuf->vmapping_counter == 0);
> -	BUG_ON(dmabuf->vmap_ptr != vaddr);
> +	BUG_ON(!dma_buf_map_is_vaddr(&dmabuf->vmap_ptr, vaddr));
>   
>   	mutex_lock(&dmabuf->lock);
>   	if (--dmabuf->vmapping_counter == 0) {
>   		if (dmabuf->ops->vunmap)
>   			dmabuf->ops->vunmap(dmabuf, vaddr);
> -		dmabuf->vmap_ptr = NULL;
> +		dma_buf_map_clear(&dmabuf->vmap_ptr);
>   	}
>   	mutex_unlock(&dmabuf->lock);
>   }
> diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
> new file mode 100644
> index 000000000000..d4b1bb3cc4b0
> --- /dev/null
> +++ b/include/linux/dma-buf-map.h
> @@ -0,0 +1,87 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Pointer to dma-buf-mapped memory, plus helpers.
> + */
> +
> +#ifndef __DMA_BUF_MAP_H__
> +#define __DMA_BUF_MAP_H__
> +
> +#include <linux/io.h>
> +
> +/**
> + * struct dma_buf_map - Pointer to vmap'ed dma-buf memory.
> + * @vaddr_iomem:	The buffer's address if in I/O memory
> + * @vaddr:		The buffer's address if in system memory
> + * @is_iomem:		True if the dma-buf memory is located in I/O
> + *			memory, or false otherwise.
> + *
> + * Calling dma-buf's vmap operation returns a pointer to the buffer.
> + * Depending on the location of the buffer, users may have to access it
> + * with I/O operations or memory load/store operations. struct dma_buf_map
> + * stores the buffer address and a flag that signals the required access.
> + */
> +struct dma_buf_map {
> +	union {
> +		void __iomem *vaddr_iomem;
> +		void *vaddr;
> +	};
> +	bool is_iomem;
> +};
> +
> +/* API transition helper */
> +static inline bool dma_buf_map_is_vaddr(const struct dma_buf_map *map, const void *vaddr)
> +{
> +	return !map->is_iomem && (map->vaddr == vaddr);
> +}
> +
> +/**
> + * dma_buf_map_is_null - Tests for a dma-buf mapping to be NULL
> + * @map:	The dma-buf mapping structure
> + *
> + * Depending on the state of struct dma_buf_map.is_iomem, tests if the
> + * mapping is NULL.
> + *
> + * Returns:
> + * True if the mapping is NULL, or false otherwise.
> + */
> +static inline bool dma_buf_map_is_null(const struct dma_buf_map *map)
> +{
> +	if (map->is_iomem)
> +		return map->vaddr_iomem == NULL;
> +	return map->vaddr == NULL;

This looks like overkill since both pointers are part of the same union.

I suggest to test only one and add a comment.

> +}
> +
> +/**
> + * dma_buf_map_is_set - Tests is the dma-buf mapping has been set
> + * @map:	The dma-buf mapping structure
> + *
> + * Depending on the state of struct dma_buf_map.is_iomem, tests if the
> + * mapping has been set.
> + *
> + * Returns:
> + * True if the mapping is been set, or false otherwise.
> + */
> +static inline bool dma_buf_map_is_set(const struct dma_buf_map *map)
> +{
> +	return !dma_buf_map_is_null(map);
> +}
> +
> +/**
> + * dma_buf_map_clear - Clears a dma-buf mapping structure
> + * @map:	The dma-buf mapping structure
> + *
> + * Clears all fields to zero; including struct dma_buf_map.is_iomem. So
> + * mapping structures that were set to point to I/O memory are reset for
> + * system memory. Pointers are cleared to NULL. This is the default.
> + */
> +static inline void dma_buf_map_clear(struct dma_buf_map *map)
> +{
> +	if (map->is_iomem) {
> +		map->vaddr_iomem = NULL;
> +		map->is_iomem = false;
> +	} else {
> +		map->vaddr = NULL;
> +	}

Maybe just use memset() on the structure.

Christian.

> +}
> +
> +#endif /* __DMA_BUF_MAP_H__ */
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 957b398d30e5..fcc2ddfb6d18 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -13,6 +13,7 @@
>   #ifndef __DMA_BUF_H__
>   #define __DMA_BUF_H__
>   
> +#include <linux/dma-buf-map.h>
>   #include <linux/file.h>
>   #include <linux/err.h>
>   #include <linux/scatterlist.h>
> @@ -309,7 +310,7 @@ struct dma_buf {
>   	const struct dma_buf_ops *ops;
>   	struct mutex lock;
>   	unsigned vmapping_counter;
> -	void *vmap_ptr;
> +	struct dma_buf_map vmap_ptr;
>   	const char *exp_name;
>   	const char *name;
>   	spinlock_t name_lock;

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