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

From	Wolfram Sang <wsa@the-dreams.de>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling
Date	2017-06-06 12:30 +0200
Message-ID	<tPjUl-7rj-7@gated-at.bofh.it> (permalink)
References	<tPiYi-6PD-9@gated-at.bofh.it> <tPiYi-6PD-17@gated-at.bofh.it> <tPjAZ-7ik-7@gated-at.bofh.it>
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> > +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 (FIXME: > 8 byte?).
> 
> So you expect drivers to fall back to PIO automatically if the buffer is
> not DMA safe.  Sounds good to me.

Yes, I strongly recommend that. Otherwise, drivers can always deal with
bounce buffers on their own.

> However, your check for a DMA-capable buffer is invoked only if
> CONFIG_DMA_API_DEBUG is enabled:

is *NOT* enabled!

> 
>     #if !defined(CONFIG_DMA_API_DEBUG)
>            if (!virt_addr_valid(msg->buf) || object_is_on_stack(msg->buf)) {
>                    pr_debug("msg buffer to 0x%04x might not be DMA capable\n",
>                             msg->addr);
>                    return -EFAULT;
>            }
>     #endif
> 

The #if block is there because DMA_API_DEBUG does a lot more, but if the
check in the helper is enabled, the core will fall back to PIO and you
won't get the additional info from DMA_API_DEBUG.

I think this needs a comment :)

Now OK?

Regards,

   Wolfram

Thread

[RFC PATCH 2/4] i2c: add docs to clarify DMA handling Wolfram Sang <wsa+renesas@sang-engineering.com> - 2017-06-06 11:30 +0200
  Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Geert Uytterhoeven <geert@linux-m68k.org> - 2017-06-06 12:10 +0200
    Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Wolfram Sang <wsa@the-dreams.de> - 2017-06-06 12:30 +0200
      Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Geert Uytterhoeven <geert@linux-m68k.org> - 2017-06-06 12:40 +0200
        Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Wolfram Sang <wsa@the-dreams.de> - 2017-06-06 13:00 +0200
          Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Geert Uytterhoeven <geert@linux-m68k.org> - 2017-06-06 13:20 +0200
            Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Wolfram Sang <wsa@the-dreams.de> - 2017-06-06 13:30 +0200
              Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling Geert Uytterhoeven <geert@linux-m68k.org> - 2017-06-06 17:30 +0200

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