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diff --git a/Documentation/driver-api/media/drivers/bttv-devel.rst b/Documentation/driver-api/media/drivers/bttv-devel.rst new file mode 100644 index 000000000..0885a0456 --- /dev/null +++ b/Documentation/driver-api/media/drivers/bttv-devel.rst @@ -0,0 +1,116 @@ +.. SPDX-License-Identifier: GPL-2.0 + +The bttv driver +=============== + +bttv and sound mini howto +------------------------- + +There are a lot of different bt848/849/878/879 based boards available. +Making video work often is not a big deal, because this is handled +completely by the bt8xx chip, which is common on all boards. But +sound is handled in slightly different ways on each board. + +To handle the grabber boards correctly, there is a array tvcards[] in +bttv-cards.c, which holds the information required for each board. +Sound will work only, if the correct entry is used (for video it often +makes no difference). The bttv driver prints a line to the kernel +log, telling which card type is used. Like this one:: + + bttv0: model: BT848(Hauppauge old) [autodetected] + +You should verify this is correct. If it isn't, you have to pass the +correct board type as insmod argument, ``insmod bttv card=2`` for +example. The file Documentation/admin-guide/media/bttv-cardlist.rst has a list +of valid arguments for card. + +If your card isn't listed there, you might check the source code for +new entries which are not listed yet. If there isn't one for your +card, you can check if one of the existing entries does work for you +(just trial and error...). + +Some boards have an extra processor for sound to do stereo decoding +and other nice features. The msp34xx chips are used by Hauppauge for +example. If your board has one, you might have to load a helper +module like ``msp3400`` to make sound work. If there isn't one for the +chip used on your board: Bad luck. Start writing a new one. Well, +you might want to check the video4linux mailing list archive first... + +Of course you need a correctly installed soundcard unless you have the +speakers connected directly to the grabber board. Hint: check the +mixer settings too. ALSA for example has everything muted by default. + + +How sound works in detail +~~~~~~~~~~~~~~~~~~~~~~~~~ + +Still doesn't work? Looks like some driver hacking is required. +Below is a do-it-yourself description for you. + +The bt8xx chips have 32 general purpose pins, and registers to control +these pins. One register is the output enable register +(``BT848_GPIO_OUT_EN``), it says which pins are actively driven by the +bt848 chip. Another one is the data register (``BT848_GPIO_DATA``), where +you can get/set the status if these pins. They can be used for input +and output. + +Most grabber board vendors use these pins to control an external chip +which does the sound routing. But every board is a little different. +These pins are also used by some companies to drive remote control +receiver chips. Some boards use the i2c bus instead of the gpio pins +to connect the mux chip. + +As mentioned above, there is a array which holds the required +information for each known board. You basically have to create a new +line for your board. The important fields are these two:: + + struct tvcard + { + [ ... ] + u32 gpiomask; + u32 audiomux[6]; /* Tuner, Radio, external, internal, mute, stereo */ + }; + +gpiomask specifies which pins are used to control the audio mux chip. +The corresponding bits in the output enable register +(``BT848_GPIO_OUT_EN``) will be set as these pins must be driven by the +bt848 chip. + +The ``audiomux[]`` array holds the data values for the different inputs +(i.e. which pins must be high/low for tuner/mute/...). This will be +written to the data register (``BT848_GPIO_DATA``) to switch the audio +mux. + + +What you have to do is figure out the correct values for gpiomask and +the audiomux array. If you have Windows and the drivers four your +card installed, you might to check out if you can read these registers +values used by the windows driver. A tool to do this is available +from http://btwincap.sourceforge.net/download.html. + +You might also dig around in the ``*.ini`` files of the Windows applications. +You can have a look at the board to see which of the gpio pins are +connected at all and then start trial-and-error ... + + +Starting with release 0.7.41 bttv has a number of insmod options to +make the gpio debugging easier: + + ================= ============================================== + bttv_gpio=0/1 enable/disable gpio debug messages + gpiomask=n set the gpiomask value + audiomux=i,j,... set the values of the audiomux array + audioall=a set the values of the audiomux array (one + value for all array elements, useful to check + out which effect the particular value has). + ================= ============================================== + +The messages printed with ``bttv_gpio=1`` look like this:: + + bttv0: gpio: en=00000027, out=00000024 in=00ffffd8 [audio: off] + + en = output _en_able register (BT848_GPIO_OUT_EN) + out = _out_put bits of the data register (BT848_GPIO_DATA), + i.e. BT848_GPIO_DATA & BT848_GPIO_OUT_EN + in = _in_put bits of the data register, + i.e. BT848_GPIO_DATA & ~BT848_GPIO_OUT_EN |