On Thursday 03 December 2015 20:49:06 Pingbo Wen wrote:
> > 在 2015年12月1日，18:47，Arnd Bergmann <a...@arndb.de> 写道：
> > On Tuesday 01 December 2015 16:34:00 Pingbo Wen wrote:
> >> We can force kernel using monotonic time in EV_IF_LEGACY interface, and
> >> making input_event independent from time_t(after evdev has converted to
> >> input_value, it’s easy to do that), but that also imply userspace
> >> must change their code to fit this change. If changing userspace code is
> >> a mandatory option, why not to force them do a complete conversion?
> >
> > Most user space programs won't care, as they don't even look at the tv_sec
> > portion, and the goal is to avoid having to change them.
> >
> > There is still an open question to how exactly we want to get user space
> > to change.
> >
> > We could do some compile-time trick by having a union in struct input_event
> > and mark the existing timeval part as deprecated, so we flag any use of the
> > 32-bit tv_sec member, such as:
> >
> > struct input_event {
> > #if !defined(__KERNEL__) && __TIME_T_BITS == __BITS_PER_LONG
> > struct timeval time;
>
> > #else
> > struct {
> > union {
> > __u32 tv_sec __attribute__((deprecated));
> > __u32 tv_sec_monotonic;
> > };
> > __s32 tv_usec;
> > } time;
> > #endif
> > __u16 type;
> > __u16 code;
> > __s32 value;
> > };
>
> I have one question here, if userspace use this structure, all helper
> functions
> of timeval will not work. And userspace need to write extra helper function
> for
> this fake timeval. This just create an another urgly time structure.
Correct, this is a useful side-effect of the change: any user space access to
the event->time member that assumes it's a timeval will cause a compile-time
warning or error (depending on the access), which helps us identify the
broken code and fix it to use monotonic times as well as access the right
struct members.
> And this method also forces most of old binaries to compile with new libc,
> adjust
> their codes with new fake time structure.
>
> Besides, I get an idea to combine your structure with input_composite_event:
>
> union {
> struct {
> __s32 tv_usec;
> __s32 tv_sec;
> };
> __s64 time;
> } time;
>
> I prefer to use a single s64 timestamp, if our goal is to remove timeval from
> kernel.
We can't really remove this instance of timeval anyway, so adding an __s64
member
here is not all that helpful. We should use __s64 nanoseconds for new interfaces
like this, but I see no reason to change the one we have here.
Arnd
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