Subject: Re: linux-next: build failure after merge of the akpm tree

From: Takashi Iwai <[email-masked]>
Date: Sat, 16 Jan 2016 09:51:29 +0100

On Sat, 16 Jan 2016 00:14:01 +0100,
Andrew Morton wrote:
> 
> On Sat, 16 Jan 2016 10:00:47 +1100 Stephen Rothwell <[email-masked]> wrote:
> 
> > Hi Andrew,
> > > +#define SNDRV_COMPRESS_GET_CODEC_CAPS	_IOWR('C', 0x11, struct snd_compr_caps)
> > >  #define SNDRV_COMPRESS_SET_PARAMS	_IOW('C', 0x12, struct snd_compr_params)
> > >  #define SNDRV_COMPRESS_GET_PARAMS	_IOR('C', 0x13, struct snd_codec)
> > >  #define SNDRV_COMPRESS_SET_METADATA	_IOW('C', 0x14,\
> > > 
> > > then it compiles OK.  So there's something special about `struct
> > > snd_compr_codec_caps' which is confusing the compiler.
> > 
> > Its very big (hint, I think it is bigger than 1 << 13).
> 
> ah-hah.
> 
> sizeof(struct snd_compr_codec_caps) is 11144 on x86_64
> allyesconfig, but x86_64 uses (1<<14), not (1<<13) - x86_64 has more
> space in the ioctl payload.
> 
> 
> So.  Sound guys: your struct snd_compr_codec_caps is too large for
> powerpc ioctls.

Ouch, this was what I was concerned.  I thought we did calculate it at
the time writing API, alas, we failed.  We need to do math drills
again...

There are a few ways to fix this, but all are not comfortable.

A. Disable compress API for powerpc.

B. Disable this ioctl for powerpc and keep it for others while
   providing yet a new ioctl for the similar functionality.

  B1. Pass the same struct via a pointer: this needs a special
      compat_ioctl handling
  B2. Reduce the size of struct snd_compr_codec_caps, e.g. set
      MAX_NUM_CODEC_DESCRIPTORS to 16.

C. What else?

Takashi