Re: [PATCH v5 09/10] dma-buf-map: Add memcpy and pointer-increment interfaces

[Linus Walleij <linus.walleij@xxxxxxxxxx>]

Overall I like this, just an inline question:

On Tue, Oct 20, 2020 at 2:20 PM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:

> To do framebuffer updates, one needs memcpy from system memory and a
> pointer-increment function. Add both interfaces with documentation.

(...)
> +/**
> + * dma_buf_map_memcpy_to - Memcpy into dma-buf mapping
> + * @dst:       The dma-buf mapping structure
> + * @src:       The source buffer
> + * @len:       The number of byte in src
> + *
> + * Copies data into a dma-buf 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 dma_buf_map_memcpy_to(struct dma_buf_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);
> +}

Are these going to be really big memcpy() operations?

Some platforms have DMA offload engines that can perform memcpy(),
drivers/dma, include/linux/dmaengine.h
especially if the CPU doesn't really need to touch the contents
and flush caches etc.
An example exist in some MTD drivers that move large quantities of
data off flash memory like this:
drivers/mtd/nand/raw/cadence-nand-controller.c

Notice that DMAengine and DMAbuf does not have much in common,
the names can be deceiving.

The value of this varies with the system architecture. It is not just
a question about performance but also about power and the CPU
being able to do other stuff in parallel for large transfers. So *when*
to use this facility to accelerate memcpy() is a delicate question.

What I'm after here is if these can be really big, do we want
(in the long run, not now) open up to the idea to slot in
hardware-accelerated memcpy() here?

Yours,
Linus Walleij