Documentation: update nRF91 doc

Documentation/platforms/arm/nrf91/index.rst

@@ -10,6 +10,98 @@ Modem Support
 
 Modem is supported in the nRF91 using Nordic's `Modem library <https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/nrfxlib/nrf_modem/README.html>`_.
 
+Supported modem features:
+
+=============  ========= ===============
+Modem feature  Support   Notes
+=============  ========= ===============
+AT             Yes       ``/dev/modem``
+Socket         Yes       via ``usrsock``
+SIOCLTECMD     Partial
+GNSS           Yes
+AGPS           No
+Bootloader     No
+Delta DFU      No
+Modem Trace    No
+=============  ========= ===============
+
+Peripheral Support
+==================
+
+The following list indicates peripherals supported in NuttX:
+
+==========  ======= =================
+Peripheral  Support Notes
+==========  ======= =================
+CRUPTOCELL  No
+DPPI        No
+EGU         No
+GPIO        Yes
+GPIOTE      Yes
+IPC         Yes      nrfx compatible
+I2S         No
+KMU         No
+PDM         No
+PWM         Yes
+RTC         Yes
+SPIM        Yes
+SPIS        No
+SPU         Yes
+TIMER       Yes
+TWIM        Yes
+TWIS        No
+UART        Yes
+UARTE       No
+WDT         No
+==========  ======= =================
+
+I2C
+---
+
+I2C is supported both in polling and interrupt mode (via EasyDMA).
+
+.. note:: The I2C peripheral does not support sending two transfers without sending
+   a START nor RSTART. For this reason, this is supported via an internal buffer where
+   messages will be first copied to and sent together.
+
+The lower-half of I2C bus is initialized by :c:func:`nrf91_i2cbus_initialize`.
+
+SPI
+---
+
+SPI is supported both in polling and interrupt-based (via EasyDMA) mode. The latter
+supports arbitrarily long transfers using Nordic's list-mode EasyDMA (intermediate
+transfers are currently still manually started).
+
+It is possible to use SPI without either MOSI/MISO pin defined by simply not providing
+the relevant ``BOARD_SPI*_MISO/MOSI_PIN`` definition.
+
+This implementation support power management hooks, which will disable SPI peripheral when
+entering either SLEEP or STANDBY modes and reconfigure it when going back to NORMAL mode.
+
+UART
+----
+
+UART is implemented using polling. UARTE EasyDMA feature is not yet supported.
+This may introduce a large number of interrupts which may be undesirable.
+
+PWM
+---
+
+PWM is supported via standard driver. This means that more advanced features such as
+complex sequences or waveform modes are not yet supported.
+
+TIMER
+-----
+
+The TIMER peripheral is exposed as standard timer.
+
+RTC
+---
+
+The RTC peripheral is exposed as a standard timer, since it is really a low-power
+timer, without any date handling capabilities.
+
 Tool Issues
 ===========