Re: [PATCH 01/19] dma-buf-map: Add read/write helpers

Thomas Zimmermann
2022-01-27T14:26:47Z

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Hi

Am 26.01.22 um 21:36 schrieb Lucas De Marchi:
> In certain situations it's useful to be able to read or write to an
> offset that is calculated by having the memory layout given by a struct
> declaration. Usually we are going to read/write a u8, u16, u32 or u64.
>
> Add a pair of macros dma_buf_map_read_field()/dma_buf_map_write_field()
> to calculate the offset of a struct member and memcpy the data from/to
> the dma_buf_map. We could use readb, readw, readl, readq and the write*
> counterparts, however due to alignment issues this may not work on all
> architectures. If alignment needs to be checked to call the right
> function, it's not possible to decide at compile-time which function to
> call: so just leave the decision to the memcpy function that will do
> exactly that on IO memory or dereference the pointer.
>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: linux-media@vger.kernel.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
> ---
> include/linux/dma-buf-map.h | 81 +++++++++++++++++++++++++++++++++++++
> 1 file changed, 81 insertions(+)
>
> diff --git a/include/linux/dma-buf-map.h b/include/linux/dma-buf-map.h
> index 19fa0b5ae5ec..65e927d9ce33 100644
> --- a/include/linux/dma-buf-map.h
> +++ b/include/linux/dma-buf-map.h
> @@ -6,6 +6,7 @@
> #ifndef __DMA_BUF_MAP_H__
> #define __DMA_BUF_MAP_H__
>
> +#include <linux/kernel.h>
> #include <linux/io.h>
> #include <linux/string.h>
>
> @@ -229,6 +230,46 @@ static inline void dma_buf_map_clear(struct dma_buf_map *map)
> }
> }
>
> +/**
> + * dma_buf_map_memcpy_to_offset - Memcpy into offset of dma-buf mapping
> + * @dst: The dma-buf mapping structure
> + * @offset: The offset from which to copy
> + * @src: The source buffer
> + * @len: The number of byte in src
> + *
> + * Copies data into a dma-buf mapping with an offset. 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 dma_buf_map_memcpy_to_offset(struct dma_buf_map *dst, size_t offset,
> + const void *src, size_t len)
> +{
> + if (dst->is_iomem)
> + memcpy_toio(dst->vaddr_iomem + offset, src, len);
> + else
> + memcpy(dst->vaddr + offset, src, len);
> +}

Please don't add a new function. Rather please add the offset parameter
to dma_buf_map_memcpy_to() and update the callers. There are only two
calls to dma_buf_map_memcpy_to() within the kernel. To make it clear
what the offset applies to, I'd call the parameter 'dst_offset'.

> +
> +/**
> + * dma_buf_map_memcpy_from_offset - Memcpy from offset of dma-buf mapping into system memory
> + * @dst: Destination in system memory
> + * @src: The dma-buf mapping structure
> + * @src: The offset from which to copy
> + * @len: The number of byte in src
> + *
> + * Copies data from a dma-buf mapping with an offset. The dest buffer is in
> + * system memory. Depending on the mapping location, the helper picks the
> + * correct method of accessing the memory.
> + */
> +static inline void dma_buf_map_memcpy_from_offset(void *dst, const struct dma_buf_map *src,
> + size_t offset, size_t len)
> +{
> + if (src->is_iomem)
> + memcpy_fromio(dst, src->vaddr_iomem + offset, len);
> + else
> + memcpy(dst, src->vaddr + offset, len);
> +}
> +

With the dma_buf_map_memcpy_to() changes, please just call this function
dma_buf_map_memcpy_from().

> /**
> * dma_buf_map_memcpy_to - Memcpy into dma-buf mapping
> * @dst: The dma-buf mapping structure
> @@ -263,4 +304,44 @@ static inline void dma_buf_map_incr(struct dma_buf_map *map, size_t incr)
> map->vaddr += incr;
> }
>
> +/**
> + * dma_buf_map_read_field - Read struct member from dma-buf mapping with
> + * arbitrary size and handling un-aligned accesses
> + *
> + * @map__: The dma-buf mapping structure
> + * @type__: The struct to be used containing the field to read
> + * @field__: Member from struct we want to read
> + *
> + * Read a value from dma-buf mapping calculating the offset and size: this assumes
> + * the dma-buf mapping is aligned with a a struct type__. A single u8, u16, u32
> + * or u64 can be read, based on the offset and size of type__.field__.
> + */
> +#define dma_buf_map_read_field(map__, type__, field__) ({ \
> + type__ *t__; \
> + typeof(t__->field__) val__; \
> + dma_buf_map_memcpy_from_offset(&val__, map__, offsetof(type__, field__), \
> + sizeof(t__->field__)); \
> + val__; \
> +})
> +
> +/**
> + * dma_buf_map_write_field - Write struct member to the dma-buf mapping with
> + * arbitrary size and handling un-aligned accesses
> + *
> + * @map__: The dma-buf mapping structure
> + * @type__: The struct to be used containing the field to write
> + * @field__: Member from struct we want to write
> + * @val__: Value to be written
> + *
> + * Write a value to the dma-buf mapping calculating the offset and size.
> + * A single u8, u16, u32 or u64 can be written based on the offset and size of
> + * type__.field__.
> + */
> +#define dma_buf_map_write_field(map__, type__, field__, val__) ({ \
> + type__ *t__; \
> + typeof(t__->field__) val____ = val__; \
> + dma_buf_map_memcpy_to_offset(map__, offsetof(type__, field__), \
> + &val____, sizeof(t__->field__)); \
> +})

As the original author of this file, I feel like this shouldn't be here.
At least not until we have another driver using that pattern.

Best regards
Thomas

> +
> #endif /* __DMA_BUF_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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