Re: [RFC PATCH 2/7] sound: core: Avoid using timespec for struct snd_pcm_status

[Arnd Bergmann]

On Fri, Sep 22, 2017 at 12:49 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> On Fri, 22 Sep 2017 12:14:55 +0200,
> Arnd Bergmann wrote:
>> The kernel uses timespec64 internally, which is defined as
>> "{ s64 tv_sec; long tv_nsec };", so this has the padding
>> in a different place on big-endian architectures, and has a
>> different alignment and size on i386. We plan to introduce
>> a 'struct __kernel_timespec' that is compatible with the
>> __64_BIT_TIME_T version of the user timespec, but that
>> doesn't exist yet.
>>
>> If you prefer, we can probably introduce it now with Baolin's
>> series, I think Deepa was planning to post a patch to add
>> it soon anyway.
>
> Yes, this sounds like a saner solution than defining the own timespec
> at each place individually.  Then we can have better conversion
> macros, too, I suppose.

Thinking about it again, we unfortunately can't use
__kernel_timespec until after all 32-bit architectures have
been converted to use the new syscalls that we still need
to introduce: In the meantime the plan is that '__kernel_timespec'
is an alias for the usual 'timespec' in user space and may still
be 32-bit wide.

I definitely agree that open-coding 'struct { s64 tv_sec;
s64 tv_nsec}' in a dozen locations is not overly helpful.

I suggested a different alternative in my reply to patch 3/7.
Can you have a look at that? The idea would be that we just
flatten all the structures in the ioctl implementation and make
the structure definition very explicit using u32/s32/u64/s64
members with no implied padding or architecture-specific
types.

> And, if we have kernel_timespec (or kernel_timespec64 or such), can
> this deprecate the existing timespec64 usages, too?  I see that
> timespec64 is internal only, so consolidation would be beneficial for
> code simplification.

Our current longterm plan is to only use __kernel_timespec on the
ABI side, where we have to watch out for the tricky conversion of
tv_nsec: Any timespec copied from a 32-bit process into the kernel
must ignore the upper half of the nanoseconds, while copying the
same structure from a 64-bit process must return an error if the
64-bit nanoseconds are larger than 999999999. When copying a
timespec into user space, we have to be careful to zero the upper
half of tv_nsec to avoid leaking uninitialized kernel data.

Inside of the kernel, we can ignore those constraints, so I'd keep
using the timespec64. We certainly don't want to use the 64-bit
nanoseconds field for internal uses on 32-bit kernels, as that
would introduce expensive 64-bit arithmetic in a lot of places
that don't need it.

My hope is also that we can eventually deprecate any use of the
plain 'timespec' in the kernel: all internal users should migrate
to timespec64 (one at a time, so we can properly review the
changes), and the uapi uses should either have the 64-bit
version of __kernel_timespec, or use compat_timespec once
that becomes usable on 32-bit architectures.

        Arnd