From mboxrd@z Thu Jan  1 00:00:00 1970
From: Clemens Ladisch <clemens@ladisch.de>
Subject: Re: [WIP PATCH] Scarlett mixer interface for 6i6, 18i6,
	18i8 and 18i20.
Date: Mon, 13 Oct 2014 13:38:55 +0200
Message-ID: <543BB9CF.7010806@ladisch.de>
References: <1412694986-2537-1-git-send-email-david.henningsson@canonical.com>
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To: Tobias Hoffmann <smilingthax@googlemail.com>, David Henningsson <david.henningsson@canonical.com>
Cc: tiwai@suse.de, alsa-devel@alsa-project.org, robin@gareus.org
List-Id: alsa-devel@alsa-project.org

Tobias Hoffmann wrote:
> On 07/10/14 17:16, David Henningsson wrote:
>> +    err = set_ctl_urb2(mixer->chip, UAC2_CS_CUR, 0x0100, 0x29, "\x80\xbb\x00\x00", 4);
>
> There have been two reports of users that had to replace the set_ctl_urb2 line with:
>
>    char buffer[4] = { '\x80', '\xbb', '\x00', '\x00' };
>    err = set_ctl_urb2(mixer->chip, UAC2_CS_CUR, 0x0100, 0x29, buffer, 4);
>
> otherwise they would get a kernel oops, e.g.:
>
>   BUG: unable to handle kernel paging request at ffffffffa0b43440
>   IP: [<ffffffff8136f3c2>] memcpy+0x12/0x110
>   ...
>   Call Trace:
>     [<ffffffffa0b311d2>] ? snd_usb_ctl_msg+0xc2/0x160 [snd_usb_audio]
>     [<ffffffffa0b3b7aa>] set_ctl_urb2+0xaa/0x100 [snd_usb_audio]
>     [<ffffffffa0b3c9fa>] scarlett_mixer_controls+0x28a/0x4c0 [snd_usb_audio]

Strange.

> Maybe it has to do with a particular compiler, or some kernel config
> setting that places the bytes "somewhere else" when they are passed as
> string?

Buffers on the stack or in modules cannot be used for DMA on some
architectures, but that is why snd_usb_ctl_msg() creates a copy of the
data.

This crash is inside memcpy(), where this memory type cannot make any
difference.

Please show the entire oops.


Regards,
Clemens