Re: [PATCH 2/4] i2c: add docs to clarify DMA handling

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Wolfram,

On Thu, Jun 15, 2017 at 8:30 PM, Wolfram Sang
<wsa+renesas@sang-engineering.com> wrote:
> --- /dev/null
> +++ b/Documentation/i2c/DMA-considerations
> @@ -0,0 +1,37 @@
> +Linux I2C and DMA
> +-----------------
> +
> +Given that I2C is a low-speed bus where largely small messages are transferred,
> +it is not considered a prime user of DMA access. At this time of writing, only
> +10% of I2C bus master drivers have DMA support implemented. And the vast
> +majority of transactions are so small that setting up DMA for it will likely
> +add more overhead than a plain PIO transfer.
> +
> +Therefore, it is *not* mandatory that the buffer of an I2C message is DMA safe.
> +It does not seem reasonable to apply additional burdens when the feature is so
> +rarely used. However, it is recommended to use a DMA-safe buffer if your
> +message size is likely applicable for DMA. Most drivers have this threshold
> +around 8 bytes. As of today, this is mostly an educated guess, however.
> +
> +To support this scenario, drivers wishing to implement DMA can use helper
> +functions from the I2C core. One checks if a message is DMA capable in terms of
> +size and memory type. It can optionally also create a bounce buffer:
> +
> +       i2c_check_msg_for_dma(msg, threshold, &bounce_buf);

Obviously the return value must be checked before proceeding.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds