get info in a loop from a sysfs entry

get info in a loop from a sysfs entry

[Wouter Simons]

Hi all,

This might be a silly question, but I want to make sure I understand
things correctly.

I have a driver with a sysfs entry to get the next data sample every
time I read the file. Used like below it works wonderful:

# cat next
0x15814
# cat next
0x1682B

The last 12 bits are the sample and the first (20) bits are the channel
the sample is from (some ADC hardware board with 24 inputs).

Now I have some C code that will loop periodically to collect the
samples and do some magic with them and I was hoping I could simply keep
a FILE * open with a loop like this:

for (i = 0; i < count; i++) {
	if (fscanf(fd_next, "0x%X", &sample) != 1) { /* No data */
		continue;
	}
	channel = sample >> 12;
	adc_data = sample & 0xFFF;
	if (channel > 23) { /* Sample out of range */
		error_helper("ADC sample out of range", 0x32000010, 0);
		continue;
	}
	a_in[channel] = adc_data;
	fseek(fd_next, 0, SEEK_SET);
}

The problem is that after storing 1 sample, the next samples are never
getting updated. I suppose the last line (fseek) does not cause the
sysfs function to be called again as I hoped.

But the sysfs documentation in the kernel says:
If userspace seeks back to zero or does a pread(2) with an offset of '0'
the show() method will be called again, rearmed, to fill the buffer.

Can anyone tell me what I might be doing wrong?

Wouter

get info in a loop from a sysfs entry

[Wouter Simons]

On 01/04/2011 11:48 AM, Wouter Simons wrote:
> Hi all,
> 
> This might be a silly question, but I want to make sure I understand
> things correctly.
> 
> I have a driver with a sysfs entry to get the next data sample every
> time I read the file. Used like below it works wonderful:
> 
> # cat next
> 0x15814
> # cat next
> 0x1682B
> 
> The last 12 bits are the sample and the first (20) bits are the channel
> the sample is from (some ADC hardware board with 24 inputs).
> 
> Now I have some C code that will loop periodically to collect the
> samples and do some magic with them and I was hoping I could simply keep
> a FILE * open with a loop like this:
> 
> for (i = 0; i < count; i++) {
> 	if (fscanf(fd_next, "0x%X", &sample) != 1) { /* No data */
> 		continue;
> 	}
> 	channel = sample >> 12;
> 	adc_data = sample & 0xFFF;
> 	if (channel > 23) { /* Sample out of range */
> 		error_helper("ADC sample out of range", 0x32000010, 0);
> 		continue;
> 	}
> 	a_in[channel] = adc_data;
> 	fseek(fd_next, 0, SEEK_SET);
> }
> 
> The problem is that after storing 1 sample, the next samples are never
> getting updated. I suppose the last line (fseek) does not cause the
> sysfs function to be called again as I hoped.
> 
> But the sysfs documentation in the kernel says:
> If userspace seeks back to zero or does a pread(2) with an offset of '0'
> the show() method will be called again, rearmed, to fill the buffer.
> 
> Can anyone tell me what I might be doing wrong?
> 
I believe I have found the issue. The older kernel my driver is
currently running on (a 2.6.21 version maintained by the hardware
supplier) does not seem to rearm the show method on seek to 0.

crap...

Wouter

get info in a loop from a sysfs entry

[Greg KH]

On Tue, Jan 04, 2011 at 01:46:38PM +0100, Wouter Simons wrote:
> I believe I have found the issue. The older kernel my driver is
> currently running on (a 2.6.21 version maintained by the hardware
> supplier) does not seem to rearm the show method on seek to 0.

2.6.21 is so obsolete and old and insecure it's not funny.  Please use a
modern kernel version.

good luck,

greg k-h

get info in a loop from a sysfs entry

[Wouter Simons]

On 01/04/2011 02:56 PM, Greg KH wrote:
> On Tue, Jan 04, 2011 at 01:46:38PM +0100, Wouter Simons wrote:
>> I believe I have found the issue. The older kernel my driver is
>> currently running on (a 2.6.21 version maintained by the hardware
>> supplier) does not seem to rearm the show method on seek to 0.
> 
> 2.6.21 is so obsolete and old and insecure it's not funny.  Please use a
> modern kernel version.
> 

Yes, I know. It is a shame that I am currently forced to use this. The
problem is that the board that I am using is not supported in newer
kernel versions. There have been people that tried to put it into the
mainline kernel, but that is now basically unmaintained code and I
cannot compile a recent kernel for my platform.

The long-term solution is to work to integrate required drivers into the
mainline, but I work for a commercial company and we are launching a
system soon which is running in an embedded style with this platform.

The system is a TS-7800 arm based SBC from http://embeddedarm.com, the
hardware is good, but unfortunately they have chosen to take a kernel
version and make their own fork without making sure it gets updated. The
ts7800 platform support in the current kernel does not appear to work,
or I am too dumb to make it work ;-) The only good thing about this is
that I do not have to run this board connected to the internet or any
other untrusted source directly.

Hopefully, I will get the time required this year to update the required
components to the current kernel version and get it into the ARM arch,
but honestly, I started developing a few drivers for this board and am
not a linux guru (yet) so it is a daunting task to take up on my own.

So yes I would like to move to a newer kernel, but it is not trivial.

best regards, Wouter

get info in a loop from a sysfs entry

[Dave Hylands]

Hi Wouter,

On Tue, Jan 4, 2011 at 2:48 AM, Wouter Simons <lkml@woutersimons.org> wrote:
> Hi all,
>
> This might be a silly question, but I want to make sure I understand
> things correctly.
>
> I have a driver with a sysfs entry to get the next data sample every
> time I read the file. Used like below it works wonderful:
>
> # cat next
> 0x15814
> # cat next
> 0x1682B
>
> The last 12 bits are the sample and the first (20) bits are the channel
> the sample is from (some ADC hardware board with 24 inputs).
>
> Now I have some C code that will loop periodically to collect the
> samples and do some magic with them and I was hoping I could simply keep
> a FILE * open with a loop like this:
>
> for (i = 0; i < count; i++) {
> ? ? ? ?if (fscanf(fd_next, "0x%X", &sample) != 1) { /* No data */
> ? ? ? ? ? ? ? ?continue;
> ? ? ? ?}

I would change this to use unbuffered I/O routines (i.e.
open/read/lseek/close) and use sscanf rather than fscanf.
fopen/fread/fseek/fclose use buffering by default.

That would eliminate any buffering that the user side runtime library
is doing. I suspect that because the data is buffered by the FILE *
routines, even doing the seek is just re-returning the data that was
read the first time around.

Dave Hylands

get info in a loop from a sysfs entry

[Wouter Simons]

On 01/04/2011 04:52 PM, Dave Hylands wrote:
> On Tue, Jan 4, 2011 at 2:48 AM, Wouter Simons <lkml@woutersimons.org> wrote:
>>
>> for (i = 0; i < count; i++) {
>>        if (fscanf(fd_next, "0x%X", &sample) != 1) { /* No data */
>>                continue;
>>        }
> 
> I would change this to use unbuffered I/O routines (i.e.
> open/read/lseek/close) and use sscanf rather than fscanf.
> fopen/fread/fseek/fclose use buffering by default.
> 
> That would eliminate any buffering that the user side runtime library
> is doing. I suspect that because the data is buffered by the FILE *
> routines, even doing the seek is just re-returning the data that was
> read the first time around.

Thanks Dave, a quick test application shows that your solution works
perfectly. This saves me a lot of overhead!

Best regards,

Wouter