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| -rw-r--r-- | Documentation/media/kapi/dtv-core.rst | 279 |
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diff --git a/Documentation/media/kapi/dtv-core.rst b/Documentation/media/kapi/dtv-core.rst index ff86bf0abeae..bec7875a7e2e 100644 --- a/Documentation/media/kapi/dtv-core.rst +++ b/Documentation/media/kapi/dtv-core.rst @@ -74,6 +74,285 @@ part of their handler for :c:type:`device_driver`.\ ``resume()``. A few other optional functions are provided to handle some special cases. +Digital TV Frontend statistics +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Introduction +^^^^^^^^^^^^ + +Digital TV frontends provide a range of +:ref:`statistics <frontend-stat-properties>` meant to help tuning the device +and measuring the quality of service. + +For each statistics measurement, the driver should set the type of scale used, +or ``FE_SCALE_NOT_AVAILABLE`` if the statistics is not available on a given +time. Drivers should also provide the number of statistics for each type. +that's usually 1 for most video standards [#f1]_. + +Drivers should initialize each statistic counters with length and +scale at its init code. For example, if the frontend provides signal +strength, it should have, on its init code:: + + struct dtv_frontend_properties *c = &state->fe.dtv_property_cache; + + c->strength.len = 1; + c->strength.stat[0].scale = FE_SCALE_NOT_AVAILABLE; + +And, when the statistics got updated, set the scale:: + + c->strength.stat[0].scale = FE_SCALE_DECIBEL; + c->strength.stat[0].uvalue = strength; + +.. [#f1] For ISDB-T, it may provide both a global statistics and a per-layer + set of statistics. On such cases, len should be equal to 4. The first + value corresponds to the global stat; the other ones to each layer, e. g.: + + - c->cnr.stat[0] for global S/N carrier ratio, + - c->cnr.stat[1] for Layer A S/N carrier ratio, + - c->cnr.stat[2] for layer B S/N carrier ratio, + - c->cnr.stat[3] for layer C S/N carrier ratio. + +.. note:: Please prefer to use ``FE_SCALE_DECIBEL`` instead of + ``FE_SCALE_RELATIVE`` for signal strength and CNR measurements. + +Groups of statistics +^^^^^^^^^^^^^^^^^^^^ + +There are several groups of statistics currently supported: + +Signal strength (:ref:`DTV-STAT-SIGNAL-STRENGTH`) + - Measures the signal strength level at the analog part of the tuner or + demod. + + - Typically obtained from the gain applied to the tuner and/or frontend + in order to detect the carrier. When no carrier is detected, the gain is + at the maximum value (so, strength is on its minimal). + + - As the gain is visible through the set of registers that adjust the gain, + typically, this statistics is always available [#f2]_. + + - Drivers should try to make it available all the times, as this statistics + can be used when adjusting an antenna position and to check for troubles + at the cabling. + + .. [#f2] On a few devices, the gain keeps floating if no carrier. + On such devices, strength report should check first if carrier is + detected at the tuner (``FE_HAS_CARRIER``, see :c:type:`fe_status`), + and otherwise return the lowest possible value. + +Carrier Signal to Noise ratio (:ref:`DTV-STAT-CNR`) + - Signal to Noise ratio for the main carrier. + + - Signal to Noise measurement depends on the device. On some hardware, is + available when the main carrier is detected. On those hardware, CNR + measurement usually comes from the tuner (e. g. after ``FE_HAS_CARRIER``, + see :c:type:`fe_status`). + + On other devices, it requires inner FEC decoding, + as the frontend measures it indirectly from other parameters (e. g. after + ``FE_HAS_VITERBI``, see :c:type:`fe_status`). + + Having it available after inner FEC is more common. + +Bit counts post-FEC (:ref:`DTV-STAT-POST-ERROR-BIT-COUNT` and :ref:`DTV-STAT-POST-TOTAL-BIT-COUNT`) + - Those counters measure the number of bits and bit errors errors after + the forward error correction (FEC) on the inner coding block + (after Viterbi, LDPC or other inner code). + + - Due to its nature, those statistics depend on full coding lock + (e. g. after ``FE_HAS_SYNC`` or after ``FE_HAS_LOCK``, + see :c:type:`fe_status`). + +Bit counts pre-FEC (:ref:`DTV-STAT-PRE-ERROR-BIT-COUNT` and :ref:`DTV-STAT-PRE-TOTAL-BIT-COUNT`) + - Those counters measure the number of bits and bit errors errors before + the forward error correction (FEC) on the inner coding block + (before Viterbi, LDPC or other inner code). + + - Not all frontends provide this kind of statistics. + + - Due to its nature, those statistics depend on inner coding lock (e. g. + after ``FE_HAS_VITERBI``, see :c:type:`fe_status`). + +Block counts (:ref:`DTV-STAT-ERROR-BLOCK-COUNT` and :ref:`DTV-STAT-TOTAL-BLOCK-COUNT`) + - Those counters measure the number of blocks and block errors errors after + the forward error correction (FEC) on the inner coding block + (before Viterbi, LDPC or other inner code). + + - Due to its nature, those statistics depend on full coding lock + (e. g. after ``FE_HAS_SYNC`` or after + ``FE_HAS_LOCK``, see :c:type:`fe_status`). + +.. note:: All counters should be monotonically increased as they're + collected from the hardware. + +A typical example of the logic that handle status and statistics is:: + + static int foo_get_status_and_stats(struct dvb_frontend *fe) + { + struct foo_state *state = fe->demodulator_priv; + struct dtv_frontend_properties *c = &fe->dtv_property_cache; + + int rc; + enum fe_status *status; + + /* Both status and strength are always available */ + rc = foo_read_status(fe, &status); + if (rc < 0) + return rc; + + rc = foo_read_strength(fe); + if (rc < 0) + return rc; + + /* Check if CNR is available */ + if (!(fe->status & FE_HAS_CARRIER)) + return 0; + + rc = foo_read_cnr(fe); + if (rc < 0) + return rc; + + /* Check if pre-BER stats are available */ + if (!(fe->status & FE_HAS_VITERBI)) + return 0; + + rc = foo_get_pre_ber(fe); + if (rc < 0) + return rc; + + /* Check if post-BER stats are available */ + if (!(fe->status & FE_HAS_SYNC)) + return 0; + + rc = foo_get_post_ber(fe); + if (rc < 0) + return rc; + } + + static const struct dvb_frontend_ops ops = { + /* ... */ + .read_status = foo_get_status_and_stats, + }; + +Statistics collect +^^^^^^^^^^^^^^^^^^ + +On almost all frontend hardware, the bit and byte counts are stored by +the hardware after a certain amount of time or after the total bit/block +counter reaches a certain value (usually programable), for example, on +every 1000 ms or after receiving 1,000,000 bits. + +So, if you read the registers too soon, you'll end by reading the same +value as in the previous reading, causing the monotonic value to be +incremented too often. + +Drivers should take the responsibility to avoid too often reads. That +can be done using two approaches: + +if the driver have a bit that indicates when a collected data is ready +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + +Driver should check such bit before making the statistics available. + +An example of such behavior can be found at this code snippet (adapted +from mb86a20s driver's logic):: + + static int foo_get_pre_ber(struct dvb_frontend *fe) + { + struct foo_state *state = fe->demodulator_priv; + struct dtv_frontend_properties *c = &fe->dtv_property_cache; + int rc, bit_error; + + /* Check if the BER measures are already available */ + rc = foo_read_u8(state, 0x54); + if (rc < 0) + return rc; + + if (!rc) + return 0; + + /* Read Bit Error Count */ + bit_error = foo_read_u32(state, 0x55); + if (bit_error < 0) + return bit_error; + + /* Read Total Bit Count */ + rc = foo_read_u32(state, 0x51); + if (rc < 0) + return rc; + + c->pre_bit_error.stat[0].scale = FE_SCALE_COUNTER; + c->pre_bit_error.stat[0].uvalue += bit_error; + c->pre_bit_count.stat[0].scale = FE_SCALE_COUNTER; + c->pre_bit_count.stat[0].uvalue += rc; + + return 0; + } + +If the driver doesn't provide a statistics available check bit +%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% + +A few devices, however, may not provide a way to check if the stats are +available (or the way to check it is unknown). They may not even provide +a way to directly read the total number of bits or blocks. + +On those devices, the driver need to ensure that it won't be reading from +the register too often and/or estimate the total number of bits/blocks. + +On such drivers, a typical routine to get statistics would be like +(adapted from dib8000 driver's logic):: + + struct foo_state { + /* ... */ + + unsigned long per_jiffies_stats; + } + + static int foo_get_pre_ber(struct dvb_frontend *fe) + { + struct foo_state *state = fe->demodulator_priv; + struct dtv_frontend_properties *c = &fe->dtv_property_cache; + int rc, bit_error; + u64 bits; + + /* Check if time for stats was elapsed */ + if (!time_after(jiffies, state->per_jiffies_stats)) + return 0; + + /* Next stat should be collected in 1000 ms */ + state->per_jiffies_stats = jiffies + msecs_to_jiffies(1000); + + /* Read Bit Error Count */ + bit_error = foo_read_u32(state, 0x55); + if (bit_error < 0) + return bit_error; + + /* + * On this particular frontend, there's no register that + * would provide the number of bits per 1000ms sample. So, + * some function would calculate it based on DTV properties + */ + bits = get_number_of_bits_per_1000ms(fe); + + c->pre_bit_error.stat[0].scale = FE_SCALE_COUNTER; + c->pre_bit_error.stat[0].uvalue += bit_error; + c->pre_bit_count.stat[0].scale = FE_SCALE_COUNTER; + c->pre_bit_count.stat[0].uvalue += bits; + + return 0; + } + +Please notice that, on both cases, we're getting the statistics using the +:c:type:`dvb_frontend_ops` ``.read_status`` callback. The rationale is that +the frontend core will automatically call this function periodically +(usually, 3 times per second, when the frontend is locked). + +That warrants that we won't miss to collect a counter and increment the +monotonic stats at the right time. + +Digital TV Frontend functions and types +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + .. kernel-doc:: drivers/media/dvb-core/dvb_frontend.h |