S2API is a tag/value-based API which was proposed (and accepted) as version 5.0 of the LinuxDVB API, adding support for new digital media transmission standards including DVB-S2, and creating a way to allow easier support of future transmission standards. It was developed and tested in autumn 2008 and was merged into the Linux kernel version 2.6.28 which was released on 24 Dec 2008.
An RFC on a proposal for adding support for ISDB-T/ISDB-Tsb to DVB-API v5 was issued on 3 Aug 2009, with the intention of reaching consensus before the merge window for kernel version 2.6.32 was opened (9 Sep 2009). Anyone with an interest is encouraged to take part in the discussion at email@example.com. A pull request was issued on 14 Aug 2009: http://www.spinics.net/lists/linux-media/msg08896.html.
Following long-standing frustration amongst developers and users about the lack of progress in getting multiproto into the Linux kernel, a group of four senior developers including the maintainer proposed an alternative (Aug 29 2008) and announced that they no longer supported multiproto. See http://linuxtv.org/pipermail/linux-dvb/2008-August/028313.html. The idea had been proposed earlier http://linuxtv.org/pipermail/linux-dvb/2007-November/021618.html. See also http://article.gmane.org/gmane.linux.drivers.dvb/36643.
Technically it is quite different from multiproto. Control of the frontend is implemented using a command sequence of (tag,value) pairs to set all the required parameters and then initiate tuning. Thus it no longer depends on fixed structs to hold parameter data.
A notable advantage of the tag/value technique is that it should make it much easier to keep up with future digital media transmission standards because this will at most require the definition of additional tags (i.e. parameters) rather than a revision of the API. Transmission standards continue to multiply: some developers (and many consumers) already have hardware using standards unsupported by multiproto, such as ISDB-T and DMB-T/H. Other standards are due to go live fairly soon, e.g. DVB-T2 (Nov 2009 in UK).
A typical example of a (tag,value) pair which sets a parameter is (DTV_FREQUENCY, 11954 MHz). A tuning command sequence would typically include a full set of tuning parameters, LNB voltage and tone, and a "do tune" command. Sequences of commands may be made atomic by passing the whole command sequence in a single ioctl (for example a set of parameters followed by the tune command). Alternatively a sequence may be sent one command at a time. S2API thus offers an extra degree of flexibility over a multiproto-style approach, and the better application-to-driver communication could potentially be exploited for more advanced application functionality in in the future.
DiSEqC equipment control is unchanged in both S2API and multiproto APIs. Additional S2API tags to allow DiSEqC commands be sent with the tag/value method (as an alternative to the legacy method) could be implemented.
The development repositories are at http://linuxtv.org/hg/~stoth/s2 and http://linuxtv.org/hg/~stoth/s2-mfe. Tuning applications are here http://www.steventoth.net/linux/s2/ and here http://mercurial.intuxication.org/hg/szap-s2/. A channel scanning application is here http://mercurial.intuxication.org/hg/scan-s2. Work is progressing well on the finalization of the API design and porting of existing DVB-S2 drivers to work with it. Older drivers not requiring the new features can remain unchanged. Device support now includes:
- cx24116 demodulator driver
- Hauppauge_WinTV-HVR-4000 DVB-S/S2 PCI
- Hauppauge_WinTV-HVR-4000Lite DVB-S/S2 PCI (also known as the S2 Lite)
- TeVii S460 DVB-S/S2 PCI
- TeVii S650 DVB-S/S2 USB
- DVBWorld HD 2104 CA+CI USB Box DVB-S/S2
- SDMC DM1105 PCI chip (used in DVBWorld 2002 DVB-S & 2004 DVB-S2)
- DVBWorld DVB-S2 PCI2004C DVB-S/S2 PCI
- Silicon Labs si2109/2110 demodulator support
- DVBWorld 2102 and TeVii s600 DVB-S USB (si2109/2110 demod, Serit tuner) 
- Omicom SS4 DVB-S/S2 support (cx24116 demod)
- TBS 8920 DVB-S/S2 support (cx24116 demod)
- Prof 7300 support (cx24116 demod)
- ST stv0288 demodulator support, stb6000 tuner, DVBWorld DW2002 PCI with Earda tuner, TeVii S420 PCI
- DVBWorld USB cards support (STV0288 demod, Earda EDS-1547 tuner)
- Multiple frontend support (for example HVR4000 DVB-S/S2 + DVB-T) (repository at http://linuxtv.org/hg/~stoth/s2-mfe)
- stb0899 demodulator (TT S2-3200, S2-3600, S2-3650CI) from Igor Liplianin, tree at http://mercurial.intuxication.org/hg/s2-liplianin 
- stb0899 demodulator (TT S2-3200) from Manu Abraham, tree at http://jusst.de/hg/v4l-dvb-test 
- Mantis PCI bridge (e.g. Azurewave AD-SP400 CI (Twinhan VP-1041)) see 
The HVR4000 and the S460 have both been tested successfully using the new API, see this thread http://linuxtv.org/pipermail/linux-dvb/2008-September/028544.html.
A Kaffeine patch adding S2API and DVB-S2 support, written by one of that application's authors, has been included in the Kaffeine SVN repository. It is available here: http://linuxtv.org/pipermail/linux-dvb/2008-October/029839.html, old version here . This has been confirmed working with the s2 and s2-mfe (multiple frontend) trees.
A VDR patch adding S2API support for DVB-S, DVB-S2, DVB-T and DVB-C is available here: http://linuxtv.org/pipermail/linux-dvb/2008-October/029575.html. VDR supports officially S2API since 1.7.0, but you should use the newest 1.7.x version.
Driver code is largely independent of the API used, and hooking up an existing driver to work with a different API is quite simple (much easier than writing the driver in the first place), so this can be expected to progress rapidly. Similarly, for application code (e.g. Kaffeine, MythTV or VDR) the number of lines of code required to tune to a channel is small, and can easily be changed for a different API. For these reasons the selection of the best API may be considered independently from the available ported drivers and applications. In particular, the existing multiproto drivers will not be lost -- with simple modifications they will become S2API drivers. Since the information transmitted across the API to the driver simply reflects the parameters defined by the digital media standards (not the receiver hardware), moving a driver to a different API simply involves packing and unpacking the same information to and from a different format.
Code review and merge
Code review round 1 is summarized here http://linuxtv.org/pipermail/linux-dvb/2008-September/028658.html. The quality and quantity of technical feedback from developers are encouraging. Some developers have contributed patches for new features, device support and bug fixes. A number of the suggested changes have been incorporated and published for code review round 2. Status report here http://linuxtv.org/pipermail/linux-dvb/2008-September/028731.html.
S2API and multiproto were reviewed at the Linux Plumbers' Conference in Portland, Oregon, Wed 17th - Fri 19th September 2008: http://linuxplumbersconf.org/program/microconfs/getmc.php?mc=chehab08. See DVB round table -- New API.
An announcement regarding the conclusions of the review was made by the maintainer : http://linuxtv.org/pipermail/linux-dvb/2008-September/029155.html. S2API was selected for inclusion in kernel 2.6.28. It was intended to add support for products with the STB0899 demodulator (TT-3200 and others) before then, in addition to the device support listed above. Multiproto will therefore not be included in the Linux kernel. This decision is final.
Kernel 2.6.27 was released on 9 Oct 2008 and the merge window for kernel 2.6.28 has opened and closed. See http://lkml.org and http://www.linuxfoundation.org/en/Linux_Weather_Forecast. The pull request to merge the S2API tree, including API and drivers, was issued (http://linuxtv.org/pipermail/linux-dvb/2008-September/029313.html) and S2API was merged into v4l-dvb. A review of the multiple frontend tree (s2-mfe) was conducted and it was merged into v4l-dvb. A review of the STB0899 support has also been started -- users and developers are invited to comment in the mailing list firstname.lastname@example.org. Kernel 2.6.28 was released on 24 Dec 2008, including S2API and the drivers which were ready at that time.
S2API is not (hopefully not yet) documented in DVB API documentation, the current document version describes still the old DVB API 3.2.