lkml.org 
[lkml]   [2011]   [Dec]   [6]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC] vtunerc: virtual DVB device - is it ok to NACK driver because of worrying about possible misusage?
    On 06.12.2011 14:22, Mauro Carvalho Chehab wrote:
    > On 05-12-2011 22:07, HoP wrote:
    >>> I doubt that scan or w_scan would support it. Even if it supports, that
    >>> would mean that,
    >>> for each ioctl that would be sent to the remote server, the error
    >>> code would
    >>> take 480 ms
    >>> to return. Try to calculate how many time w_scan would work with
    >>> that. The
    >>> calculus is easy:
    >>> see how many ioctl's are called by each frequency and multiply by the
    >>> number
    >>> of frequencies
    >>> that it would be seek. You should then add the delay introduced over
    >>> streaming the data
    >>> from the demux, using the same calculus. This is the additional time
    >>> over a
    >>> local w_scan.
    >>>
    >>> A grouch calculus with scandvb: to tune into a single DVB-C
    >>> frequency, it
    >>> used 45 ioctls.
    >>> Each taking 480 ms round trip would mean an extra delay of 21.6 seconds.
    >>> There are 155
    >>> possible frequencies here. So, imagining that scan could deal with 21.6
    >>> seconds of delay
    >>> for each channel (with it doesn't), the extra delay added by it is 1
    >>> hour
    >>> (45 * 0.48 * 155).
    >>>
    >>> On the other hand, a solution like the one described by Florian would
    >>> introduce a delay of
    >>> 480 ms for the entire scan to happen, as only one data packet would be
    >>> needed to send a
    >>> scan request, and one one stream of packets traveling at 10GB/s would
    >>> bring
    >>> the answer
    >>> back.
    >>
    >> Andreas was excited by your imaginations and calculations, but not me.
    >> Now you again manifested you are not treating me as partner for
    >> discussion.
    >> Otherwise you should try to understand how-that-ugly-hack works.
    >> But you surelly didn't try to do it at all.
    >>
    >> How do you find those 45 ioctls for DVB-C tune?
    >
    > With strace. See how many ioctl's are called for each tune. Ok, perhaps
    > scandvb
    > is badly written, but if your idea is to support 100% of the
    > applications, you
    > should be prepared for badly written applications.
    >
    > $strace -e ioctl scandvb dvbc-teste
    > scanning dvbc-teste
    > using '/dev/dvb/adapter0/frontend0' and '/dev/dvb/adapter0/demux0'
    > ioctl(3, FE_GET_INFO, 0x60a640) = 0
    > initial transponder 573000000 5217000 0 5
    >>>> tune to: 573000000:INVERSION_AUTO:5217000:FEC_NONE:QAM_256
    > ioctl(3, FE_SET_FRONTEND, 0x7fff5f7f2cd0) = 0
    > ioctl(3, FE_READ_STATUS, 0x7fff5f7f2cfc) = 0
    > ioctl(3, FE_READ_STATUS, 0x7fff5f7f2cfc) = 0
    > ioctl(3, FE_READ_STATUS, 0x7fff5f7f2cfc) = 0
    > ioctl(4, DMX_SET_FILTER, 0x7fff5f7f1ad0) = 0
    > ioctl(5, DMX_SET_FILTER, 0x7fff5f7f1ad0) = 0
    > ioctl(6, DMX_SET_FILTER, 0x7fff5f7f1ad0) = 0
    > ioctl(7, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(8, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(9, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(10, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(11, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(12, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(13, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(14, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(15, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(16, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(17, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(18, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(19, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(20, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(21, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(22, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(23, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(24, DMX_SET_FILTER, 0x7fff5f7f1910) = 0
    > ioctl(4, DMX_STOP, 0x1) = 0
    > ioctl(15, DMX_STOP, 0x1) = 0
    > ioctl(11, DMX_STOP, 0x1) = 0
    > ioctl(22, DMX_STOP, 0x1) = 0
    > ioctl(17, DMX_STOP, 0x1) = 0
    > ioctl(16, DMX_STOP, 0x1) = 0

    You don't need to wait for write-only operations. Basically all demux
    ioctls are write-only. Since vtunerc is using dvb-core's software demux
    *locally*, errors for invalid arguments etc. will be returned as usual.

    What's left is one call to FE_SET_FRONTEND for each frequency to tune
    to, and one FE_READ_STATUS for each time the lock status is queried.
    Note that one may use FE_GET_EVENT instead of FE_READ_STATUS to get
    notified of status changes asynchronously if desired.

    Btw.: FE_SET_FRONTEND doesn't block either, because the driver callback
    is called from a dvb_frontend's *local* kernel thread.

    Regards,
    Andreas


    \
     
     \ /
      Last update: 2011-12-06 14:51    [W:0.029 / U:61.500 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site