Re: Laptop shock detection and harddisk protection

[Jeremy Fitzhardinge <jeremy@goop.org>]

Shem Multinymous wrote:
> Hi Tejun,
>
> On Thu, Sep 11, 2008 at 12:34 PM, Tejun Heo <tj@kernel.org> wrote:
>   
>> Hello, Shem Multinymous.
>>     
>>> Using the input device interface for the accelerometer (as done by
>>> tp_smapi's hdaps + latest hdapsd) greatly reduces the number of
>>> accelerometer-related timer interrupts on tickless kernels, as
>>> measured by powertop. With syscall polling you have the kernal polling
>>> the hardware at ~50Hz and then the userspace hdapsd polling the kernel
>>> at ~50Hz. When they're out of phase so you can get up to 100
>>> interrupts/sec. With an input device you're always at 50Hz. The phase
>>> difference also induces a small extra delay in the shock handling
>>> response.
>>>       
>> That reduction comes because input device supports poll and
>> sysfs_notify_event() does about the same thing.  The uesrland daemon
>> can just poll on a node and read data nodes when poll event on the
>> node triggeres.
>>     
>
> Agreed.
> There's another issue with the current sysfs interface, though: hdapsd
> needs to read (x,y,timestamp) tuples, whereas sysfs provides just x
> and y in separate attributes which cannot be read atomically together.
> We can add a sysfs file with "x y timestamp" readouts, though this is
> unusual for sysfs (and certainly incompatible with hwmon).
>   

Assuming timestamp is always updated when the x,y values change, you can do:

	do {
		ts = read_timestamp();
		x = read_x();
		y = read_y();
		ts2 = read_timestamp();
	} while(ts != ts2);






    J