Message-ID: <37e3c34b-6fe3-5270-2128-40158b6e0c9e@suse.de>
Date:   Fri, 28 Jan 2022 09:53:59 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Lucas De Marchi <lucas.demarchi@...el.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc:     airlied@...ux.ie, daniel.vetter@...ll.ch, christian.koenig@....com,
        srinivas.kandagatla@...aro.org, gregkh@...uxfoundation.org,
        nouveau@...ts.freedesktop.org, linux-media@...r.kernel.org
Subject: Re: [PATCH 01/14] iosys-map: Introduce renamed dma-buf-map

Hi

Am 28.01.22 um 09:36 schrieb Lucas De Marchi:
> Add a new type, struct iosys_map, to eventually replace
> struct dma_buf_map and its helpers defiend in
> include/linux/dma-buf-map.h.
> 
> This is mostly a copy of dma-buf-map with the renames in place and
> slightly different wording to avoid tying iosys_map to dma-buf: in fact
> it's just a shim layer to abstract system memory, that can be accessed
> via regular load and store, from IO memory that needs to be acessed via
> arch helpers. Over time the dma-buf-map proved useful outside of buffer
> sharing among drivers and started to be used in helper functions for
> allocation and generic use. See e.g.
> 
> 	drivers/gpu/drm/drm_gem_shmem_helper.c
> 	drivers/gpu/drm/drm_gem_framebuffer_helper.c
> 	drivers/gpu/drm/drm_fb_helper.c

Well, that was the original motivation: framebuffer memory can be 
located in system or I/O memory, and even change their location. For 
some architectures this makes difference. IIRC the framebuffer console 
crashed on sparc64 because we didn't access I/O memory in the correct 
way. Hence, we added dma_buf_map to return the type of memory from 
dma_buf_vmap/dma_buf_vunmap.  And because everything is tied together, 
we had quite a bit of churn throughout the DRM/media code. There are 
still places in DRM where we access the raw pointers within dma_buf_map. 
We need to clean this up at some point.

> 
> In the i915 driver it's also desired to share the implementation for
> integrated graphics, which uses mostly system memory, with discrete
> graphics, which may need to access IO memory.
> 
> Once all the drivers using dma_buf_map are converted, the dma_buf_map
> can be retired and iosys_map extended to cover new use cases.

Wrt the renaming: the old name isn't good and the new one isn't good 
either. But I don't have strong feelings for either of them.

> 
> Signed-off-by: Lucas De Marchi <lucas.demarchi@...el.com>
> ---
>   MAINTAINERS                 |   1 +
>   include/linux/dma-buf-map.h |   3 +
>   include/linux/iosys-map.h   | 254 ++++++++++++++++++++++++++++++++++++
>   3 files changed, 258 insertions(+)
>   create mode 100644 include/linux/iosys-map.h
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ea3e6c914384..27ebaded85f8 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -5734,6 +5734,7 @@ F:	Documentation/driver-api/dma-buf.rst
>   F:	drivers/dma-buf/
>   F:	include/linux/*fence.h
>   F:	include/linux/dma-buf*
> +F:	include/linux/iosys-map.h

If anything, I'd complain tha twe now have something in dma-buf that 
isn't obviously connected to dma-buf.

Best regards
Thomas

>   F:	include/linux/dma-resv.h
>   K:	\bdma_(?:buf|fence|resv)\b
>   
> diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
> index 19fa0b5ae5ec..4b4b2930660b 100644
> --- a/include/linux/dma-buf-map.h
> +++ b/include/linux/dma-buf-map.h
> @@ -263,4 +263,7 @@ static inline void dma_buf_map_incr(struct dma_buf_map *map, size_t incr)
>   		map->vaddr += incr;
>   }
>   
> +/* Temporary include for API migration */
> +#include <linux/iosys-map.h>
> +
>   #endif /* __DMA_BUF_MAP_H__ */
> diff --git a/include/linux/iosys-map.h b/include/linux/iosys-map.h
> new file mode 100644
> index 000000000000..ad1f08f8f97f
> --- /dev/null
> +++ b/include/linux/iosys-map.h
> @@ -0,0 +1,254 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Pointer abstraction for IO/system memory
> + */
> +
> +#ifndef __IOSYS_MAP_H__
> +#define __IOSYS_MAP_H__
> +
> +#include <linux/io.h>
> +#include <linux/string.h>
> +
> +/* Temporary include while user of dma-buf-map are converted to iosys-map */
> +#include <linux/dma-buf-map.h>
> +
> +/**
> + * DOC: overview
> + *
> + * When accessing a memory region, depending on the its location, users may have
> + * to access it with I/O operations or memory load/store operations. For
> + * example, copying to system memory could be done with memcpy(), copying to I/O
> + * memory would be done with memcpy_toio().
> + *
> + * .. code-block:: c
> + *
> + *	void *vaddr = ...; // pointer to system memory
> + *	memcpy(vaddr, src, len);
> + *
> + *	void *vaddr_iomem = ...; // pointer to I/O memory
> + *	memcpy_toio(vaddr, _iomem, src, len);
> + *
> + * The user of such pointer may not have information about the mapping of that
> + * region or may want to have a single code path to handle operations on that
> + * buffer, regardless if it's located in system or IO memory. The type
> + * :c:type:`struct iosys_map <iosys_map>` and its helpers abstract that so the
> + * buffer can be passed around to other drivers or have separate duties inside
> + * the same driver for allocation, read and write operations.
> + *
> + * Open-coding access to :c:type:`struct iosys_map <iosys_map>` is considered
> + * bad style. Rather then accessing its fields directly, use one of the provided
> + * helper functions, or implement your own. For example, instances of
> + * :c:type:`struct iosys_map <iosys_map>` can be initialized statically with
> + * IOSYS_MAP_INIT_VADDR(), or at runtime with iosys_map_set_vaddr(). These
> + * helpers will set an address in system memory.
> + *
> + * .. code-block:: c
> + *
> + *	struct iosys_map map = IOSYS_MAP_INIT_VADDR(0xdeadbeaf);
> + *
> + *	iosys_map_set_vaddr(&map, 0xdeadbeaf);
> + *
> + * To set an address in I/O memory, use iosys_map_set_vaddr_iomem().
> + *
> + * .. code-block:: c
> + *
> + *	iosys_map_set_vaddr_iomem(&map, 0xdeadbeaf);
> + *
> + * Instances of struct iosys_map do not have to be cleaned up, but
> + * can be cleared to NULL with iosys_map_clear(). Cleared mappings
> + * always refer to system memory.
> + *
> + * .. code-block:: c
> + *
> + *	iosys_map_clear(&map);
> + *
> + * Test if a mapping is valid with either iosys_map_is_set() or
> + * iosys_map_is_null().
> + *
> + * .. code-block:: c
> + *
> + *	if (iosys_map_is_set(&map) != iosys_map_is_null(&map))
> + *		// always true
> + *
> + * Instances of :c:type:`struct iosys_map <iosys_map>` can be compared for
> + * equality with iosys_map_is_equal(). Mappings that point to different memory
> + * spaces, system or I/O, are never equal. That's even true if both spaces are
> + * located in the same address space, both mappings contain the same address
> + * value, or both mappings refer to NULL.
> + *
> + * .. code-block:: c
> + *
> + *	struct iosys_map sys_map; // refers to system memory
> + *	struct iosys_map io_map; // refers to I/O memory
> + *
> + *	if (iosys_map_is_equal(&sys_map, &io_map))
> + *		// always false
> + *
> + * A set up instance of struct iosys_map can be used to access or manipulate the
> + * buffer memory. Depending on the location of the memory, the provided helpers
> + * will pick the correct operations. Data can be copied into the memory with
> + * iosys_map_memcpy_to(). The address can be manipulated with iosys_map_incr().
> + *
> + * .. code-block:: c
> + *
> + *	const void *src = ...; // source buffer
> + *	size_t len = ...; // length of src
> + *
> + *	iosys_map_memcpy_to(&map, src, len);
> + *	iosys_map_incr(&map, len); // go to first byte after the memcpy
> + */
> +
> +/**
> + * struct iosys_map - Pointer to IO/system 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 buffer is located in I/O memory, or false
> + *			otherwise.
> + */
> +#define iosys_map dma_buf_map
> +
> +/**
> + * IOSYS_MAP_INIT_VADDR - Initializes struct iosys_map to an address in system memory
> + * @vaddr_:	A system-memory address
> + */
> +#define IOSYS_MAP_INIT_VADDR(vaddr_)	\
> +	{				\
> +		.vaddr = (vaddr_),	\
> +		.is_iomem = false,	\
> +	}
> +
> +/**
> + * iosys_map_set_vaddr - Sets a iosys mapping structure to an address in system memory
> + * @map:	The iosys_map structure
> + * @vaddr:	A system-memory address
> + *
> + * Sets the address and clears the I/O-memory flag.
> + */
> +static inline void iosys_map_set_vaddr(struct iosys_map *map, void *vaddr)
> +{
> +	map->vaddr = vaddr;
> +	map->is_iomem = false;
> +}
> +
> +/**
> + * iosys_map_set_vaddr_iomem - Sets a iosys mapping structure to an address in I/O memory
> + * @map:		The iosys_map structure
> + * @vaddr_iomem:	An I/O-memory address
> + *
> + * Sets the address and the I/O-memory flag.
> + */
> +static inline void iosys_map_set_vaddr_iomem(struct iosys_map *map,
> +					     void __iomem *vaddr_iomem)
> +{
> +	map->vaddr_iomem = vaddr_iomem;
> +	map->is_iomem = true;
> +}
> +
> +/**
> + * iosys_map_is_equal - Compares two iosys mapping structures for equality
> + * @lhs:	The iosys_map structure
> + * @rhs:	A iosys_map structure to compare with
> + *
> + * Two iosys mapping structures are equal if they both refer to the same type of memory
> + * and to the same address within that memory.
> + *
> + * Returns:
> + * True is both structures are equal, or false otherwise.
> + */
> +static inline bool iosys_map_is_equal(const struct iosys_map *lhs,
> +				      const struct iosys_map *rhs)
> +{
> +	if (lhs->is_iomem != rhs->is_iomem)
> +		return false;
> +	else if (lhs->is_iomem)
> +		return lhs->vaddr_iomem == rhs->vaddr_iomem;
> +	else
> +		return lhs->vaddr == rhs->vaddr;
> +}
> +
> +/**
> + * iosys_map_is_null - Tests for a iosys mapping to be NULL
> + * @map:	The iosys_map structure
> + *
> + * Depending on the state of struct iosys_map.is_iomem, tests if the
> + * mapping is NULL.
> + *
> + * Returns:
> + * True if the mapping is NULL, or false otherwise.
> + */
> +static inline bool iosys_map_is_null(const struct iosys_map *map)
> +{
> +	if (map->is_iomem)
> +		return !map->vaddr_iomem;
> +	return !map->vaddr;
> +}
> +
> +/**
> + * iosys_map_is_set - Tests if the iosys mapping has been set
> + * @map:	The iosys_map structure
> + *
> + * Depending on the state of struct iosys_map.is_iomem, tests if the
> + * mapping has been set.
> + *
> + * Returns:
> + * True if the mapping is been set, or false otherwise.
> + */
> +static inline bool iosys_map_is_set(const struct iosys_map *map)
> +{
> +	return !iosys_map_is_null(map);
> +}
> +
> +/**
> + * iosys_map_clear - Clears a iosys mapping structure
> + * @map:	The iosys_map structure
> + *
> + * Clears all fields to zero, including struct iosys_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 iosys_map_clear(struct iosys_map *map)
> +{
> +	if (map->is_iomem) {
> +		map->vaddr_iomem = NULL;
> +		map->is_iomem = false;
> +	} else {
> +		map->vaddr = NULL;
> +	}
> +}
> +
> +/**
> + * iosys_map_memcpy_to - Memcpy into iosys mapping
> + * @dst:	The iosys_map structure
> + * @src:	The source buffer
> + * @len:	The number of byte in src
> + *
> + * Copies data into a iosys mapping. The source buffer is in system
> + * memory. Depending on the buffer's location, the helper picks the correct
> + * method of accessing the memory.
> + */
> +static inline void iosys_map_memcpy_to(struct iosys_map *dst, const void *src,
> +				       size_t len)
> +{
> +	if (dst->is_iomem)
> +		memcpy_toio(dst->vaddr_iomem, src, len);
> +	else
> +		memcpy(dst->vaddr, src, len);
> +}
> +
> +/**
> + * iosys_map_incr - Increments the address stored in a iosys mapping
> + * @map:	The iosys_map structure
> + * @incr:	The number of bytes to increment
> + *
> + * Increments the address stored in a iosys mapping. Depending on the
> + * buffer's location, the correct value will be updated.
> + */
> +static inline void iosys_map_incr(struct iosys_map *map, size_t incr)
> +{
> +	if (map->is_iomem)
> +		map->vaddr_iomem += incr;
> +	else
> +		map->vaddr += incr;
> +}
> +
> +#endif /* __IOSYS_MAP_H__ */

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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