Possible bug on libv4l read() emulation

Possible bug on libv4l read() emulation

[Pablo Baena]

I have a program where I use libv4l's read() emulation for simplicity.
But with most v4l2 webcams I've tried, I get kernel panics.

I have pics of the message if anyone cares to see them, I don't want
to flood the mailing list.

Basically, the names I see in the kernel panic from a uvcvideo card is:

uvc_queue_next_buffer
__bad_area_no_semaphore
do_page_fault

And a lot more.

TIA.

Re: Possible bug on libv4l read() emulation

[Hans de Goede]

Hi,

On 10/16/2009 12:00 AM, Pablo Baena wrote:
> I have a program where I use libv4l's read() emulation for simplicity.
> But with most v4l2 webcams I've tried, I get kernel panics.
>
> I have pics of the message if anyone cares to see them, I don't want
> to flood the mailing list.
>
> Basically, the names I see in the kernel panic from a uvcvideo card is:
>
> uvc_queue_next_buffer
> __bad_area_no_semaphore
> do_page_fault
>
> And a lot more.
>

A panic should never happen, no matter what libv4l does (as libv4l is 100%
userspace). Please submit a bug report with as much detail as possible to the
driver author.

Regards,

Hans

Re: Possible bug on libv4l read() emulation

[Pablo Baena]

I think I identified the problem, I used code extracted from:
http://www.saillard.org/linux/pwc/files/capture.c, and the code was
like:

          total_read_bytes = 0;
          do {
                read_bytes = read(fd, buffers[0].start, buffers[0].length);
                if (read_bytes < 0)
                {
                    switch (errno)
                    {
                        case EIO:
                        case EAGAIN:
                            continue;
                        default:
                            errno_exit("read");
                    }
                }
                total_read_bytes += read_bytes;
            } while (total_read_bytes < buffers[0].length);

I changed the read() call to a v4l2_read() and used libv4l, and that
piece of code was triggering the kernel panic.

To fix it, I just removed the do..while loop. I'm still trying to
figure out what's the problem with that though.

Thanks for the reply.

On Fri, Oct 16, 2009 at 2:42 PM, Hans de Goede <hdegoede@redhat.com> wrote:
> Hi,
>
> On 10/16/2009 12:00 AM, Pablo Baena wrote:
>>
>> I have a program where I use libv4l's read() emulation for simplicity.
>> But with most v4l2 webcams I've tried, I get kernel panics.
>>
>> I have pics of the message if anyone cares to see them, I don't want
>> to flood the mailing list.
>>
>> Basically, the names I see in the kernel panic from a uvcvideo card is:
>>
>> uvc_queue_next_buffer
>> __bad_area_no_semaphore
>> do_page_fault
>>
>> And a lot more.
>>
>
> A panic should never happen, no matter what libv4l does (as libv4l is 100%
> userspace). Please submit a bug report with as much detail as possible to
> the
> driver author.
>
> Regards,
>
> Hans
>



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