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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 6 Jun 2017 12:24:47 +0200
From:   Wolfram Sang <wsa@...-dreams.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Linux I2C <linux-i2c@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/4] i2c: add docs to clarify DMA handling


> > +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

Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)