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

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Date:   Fri, 10 Nov 2017 08:19:38 +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 Fri, 10 Nov 2017 00:20:10 +0100,
Arnd Bergmann wrote:
> 
> On Thu, Nov 9, 2017 at 7:11 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > 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:
> >> >
> >> > 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.
> 
> I understand your concern here and agree it's really ugly. It did take us
> many attempts to come up with this trick for other cases, so my initial
> reaction would be to use the same thing everywhere since I know
> it works,  but we can use #ifdef instead if you prefer that. I think we
> can use a single #ifdef variant to cover all cases, but I'd have to think
> about the x32 and x86-32 some more here. With this trick, we can
> make user space with new glibc use data structures that are compatible
> with 64-bit kernels and avoid the additional translation helpers:
> 
> enum {
>       SNDRV_PCM_MMAP_OFFSET_DATA = 0x00000000,
>       SNDRV_PCM_MMAP_OFFSET_CONTROL = 0x81000000,
> #if (__BITS_PER_LONG == 64) || !defined(__USE_TIME_BITS64)

Yeah, it's definitely better, more understandable!


thanks,

Takashi