linux-kernel - Re: [alsa-devel] [RFC PATCH v2 7/7] sound: core: Avoid using timespec for struct snd_timer

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Date:   Thu, 09 Nov 2017 19:11:26 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Baolin Wang <baolin.wang@...aro.org>,
        Jaroslav Kysela <perex@...ex.cz>,
        Fabian Frederick <fabf@...net.be>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        alsa-devel@...a-project.org, Vinod Koul <vinod.koul@...el.com>,
        hardik.t.shah@...el.com, guneshwor.o.singh@...el.com,
        Liam Girdwood <lgirdwood@...il.com>,
        SF Markus Elfring <elfring@...rs.sourceforge.net>,
        gudishax.kranthikumar@...el.com, Mark Brown <broonie@...nel.org>,
        Bhumika Goyal <bhumirks@...il.com>,
        Naveen M <naveen.m@...el.com>, jeeja.kp@...el.com,
        Takashi Sakamoto <o-takashi@...amocchi.jp>,
        subhransu.s.prusty@...el.com, Ingo Molnar <mingo@...nel.org>,
        Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [alsa-devel] [RFC PATCH v2 7/7] sound: core: Avoid using timespec for struct snd_timer_tread

On Thu, 09 Nov 2017 18:01:47 +0100,
Arnd Bergmann wrote:
> 
> On Thu, Nov 9, 2017 at 5:52 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > On Mon, 06 Nov 2017 17:33:26 +0100,
> 
> >> --- a/include/uapi/sound/asound.h
> >> +++ b/include/uapi/sound/asound.h
> >> @@ -306,10 +306,19 @@ typedef int __bitwise snd_pcm_state_t;
> >>
> >>  enum {
> >>         SNDRV_PCM_MMAP_OFFSET_DATA = 0x00000000,
> >> -       SNDRV_PCM_MMAP_OFFSET_STATUS = 0x80000000,
> >> +       SNDRV_PCM_MMAP_OFFSET_STATUS_OLD = 0x80000000,
> >>         SNDRV_PCM_MMAP_OFFSET_CONTROL = 0x81000000,
> >> +       SNDRV_PCM_MMAP_OFFSET_STATUS64 = 0x82000000,
> >>  };
> >>
> >> +#if __BITS_PER_LONG == 64
> >> +#define SNDRV_PCM_MMAP_OFFSET_STATUS SNDRV_PCM_MMAP_OFFSET_STATUS_OLD
> >> +#else
> >> +#define SNDRV_PCM_MMAP_OFFSET_STATUS ((sizeof(time_t) >
> >> sizeof(__kernel_long_t)) ? \
> >> +                                       SNDRV_PCM_MMAP_OFFSET_STATUS64 : \
> >> +                                       SNDRV_PCM_MMAP_OFFSET_STATUS_OLD)
> >> +#endif
> >> +
> >>  union snd_pcm_sync_id {
> >>         unsigned char id[16];
> >>         unsigned short id16[8];
> >>
> >> Does that make sense?
> >
> > Yeah, that should work.
> >
> > But can we make the flip without the dynamic sizeof() comparison but
> > some ifdef?  The above doesn't allow the usage with switch(), for
> > example.
> >
> > IOW, is there any macro indicating the 64bit user time_t?
> 
> There is a macro defined by the C library, but so far we have not
> started relying on it in kernel headers, because there is no guarantee
> that this symbol is visible before sys/time.h has been included,
> and there are some cases where it's possible to include a kernel
> header before sys/time.h.
> 
> In case of sound/asound.h, that should be no problem since we rely
> on having seen the definition on 'struct timeval' already today, and
> that must come from sys/time.h. Then we just need to make sure that
> all C libraries define the same macro.
> 
> Are you sure about the switch()/case problem? I thought that worked
> in C99, the only problem would be using the macro outside of a
> function, e.g. as initalizer for a variable

Hmm, OK it seems working.

But, honestly speaking, it's too scaring.  I'm OK if it were only in
the kernel local code.  But it's the API/ABI definition, which is
referred by user-space...

A more solid condition would be really appreciated.


thanks,

Takashi