docs/library/uctypes: Add examples and make general updates.

Examples are added to the beginning of the module docs, similarly to docs
for many other modules.

Improvements to grammar, style, and clarity. Some paragraphs are updated
with better suggestions. A warning added of the effect incorrect usage of
the module may have. Describe the fact that offset range used in one
defined structure is limited.

docs/library/uctypes.rst

@@ -11,27 +11,99 @@ module is to define data structure layout with about the same power as the
 C language allows, and then access it using familiar dot-syntax to reference
 sub-fields.
 
+.. warning::
+
+    ``uctypes`` module allows access to arbitrary memory addresses of the
+    machine (including I/O and control registers). Uncareful usage of it
+    may lead to crashes, data loss, and even hardware malfunction.
+
 .. seealso::
 
     Module :mod:`ustruct`
         Standard Python way to access binary data structures (doesn't scale
         well to large and complex structures).
 
+Usage examples::
+
+    import uctypes
+
+    # Example 1: Subset of ELF file header
+    # https://wikipedia.org/wiki/Executable_and_Linkable_Format#File_header
+    ELF_HEADER = {
+        "EI_MAG": (0x0 | uctypes.ARRAY, 4 | uctypes.UINT8),
+        "EI_DATA": 0x5 | uctypes.UINT8,
+        "e_machine": 0x12 | uctypes.UINT16,
+    }
+
+    # "f" is an ELF file opened in binary mode
+    buf = f.read(uctypes.sizeof(ELF_HEADER, uctypes.LITTLE_ENDIAN))
+    header = uctypes.struct(uctypes.addressof(buf), ELF_HEADER, uctypes.LITTLE_ENDIAN)
+    assert header.EI_MAG == b"\x7fELF"
+    assert header.EI_DATA == 1, "Oops, wrong endianness. Could retry with uctypes.BIG_ENDIAN."
+    print("machine:", hex(header.e_machine))
+
+
+    # Example 2: In-memory data structure, with pointers
+    COORD = {
+        "x": 0 | uctypes.FLOAT32,
+        "y": 4 | uctypes.FLOAT32,
+    }
+
+    STRUCT1 = {
+        "data1": 0 | uctypes.UINT8,
+        "data2": 4 | uctypes.UINT32,
+        "ptr": (8 | uctypes.PTR, COORD),
+    }
+
+    # Suppose you have address of a structure of type STRUCT1 in "addr"
+    # uctypes.NATIVE is optional (used by default)
+    struct1 = uctypes.struct(addr, STRUCT1, uctypes.NATIVE)
+    print("x:", struct1.ptr[0].x)
+
+
+    # Example 3: Access to CPU registers. Subset of STM32F4xx WWDG block
+    WWDG_LAYOUT = {
+        "WWDG_CR": (0, {
+            # BFUINT32 here means size of the WWDG_CR register
+            "WDGA": 7 << uctypes.BF_POS | 1 << uctypes.BF_LEN | uctypes.BFUINT32,
+            "T": 0 << uctypes.BF_POS | 7 << uctypes.BF_LEN | uctypes.BFUINT32,
+        }),
+        "WWDG_CFR": (4, {
+            "EWI": 9 << uctypes.BF_POS | 1 << uctypes.BF_LEN | uctypes.BFUINT32,
+            "WDGTB": 7 << uctypes.BF_POS | 2 << uctypes.BF_LEN | uctypes.BFUINT32,
+            "W": 0 << uctypes.BF_POS | 7 << uctypes.BF_LEN | uctypes.BFUINT32,
+        }),
+    }
+
+    WWDG = uctypes.struct(0x40002c00, WWDG_LAYOUT)
+
+    WWDG.WWDG_CFR.WDGTB = 0b10
+    WWDG.WWDG_CR.WDGA = 1
+    print("Current counter:", WWDG.WWDG_CR.T)
+
 Defining structure layout
 -------------------------
 
 Structure layout is defined by a "descriptor" - a Python dictionary which
 encodes field names as keys and other properties required to access them as
-associated values. Currently, uctypes requires explicit specification of
-offsets for each field. Offset are given in bytes from a structure start.
+associated values::
+
+    {
+        "field1": <properties>,
+        "field2": <properties>,
+        ...
+    }
+
+Currently, ``uctypes`` requires explicit specification of offsets for each
+field. Offset are given in bytes from the structure start.
 
 Following are encoding examples for various field types:
 
 * Scalar types::
 
     "field_name": offset | uctypes.UINT32
 
-  in other words, value is scalar type identifier ORed with field offset
+  in other words, the value is a scalar type identifier ORed with a field offset
   (in bytes) from the start of the structure.
 
 * Recursive structures::
@@ -41,52 +113,54 @@ Following are encoding examples for various field types:
         "b1": 1 | uctypes.UINT8,
     })
 
-  i.e. value is a 2-tuple, first element of which is offset, and second is
+  i.e. value is a 2-tuple, first element of which is an offset, and second is
   a structure descriptor dictionary (note: offsets in recursive descriptors
-  are relative to the structure it defines).
+  are relative to the structure it defines). Of course, recursive structures
+  can be specified not just by a literal dictionary, but by referring to a
+  structure descriptor dictionary (defined earlier) by name.
 
 * Arrays of primitive types::
 
       "arr": (offset | uctypes.ARRAY, size | uctypes.UINT8),
 
   i.e. value is a 2-tuple, first element of which is ARRAY flag ORed
   with offset, and second is scalar element type ORed number of elements
-  in array.
+  in the array.
 
 * Arrays of aggregate types::
 
     "arr2": (offset | uctypes.ARRAY, size, {"b": 0 | uctypes.UINT8}),
 
   i.e. value is a 3-tuple, first element of which is ARRAY flag ORed
-  with offset, second is a number of elements in array, and third is
-  descriptor of element type.
+  with offset, second is a number of elements in the array, and third is
+  a descriptor of element type.
 
 * Pointer to a primitive type::
 
     "ptr": (offset | uctypes.PTR, uctypes.UINT8),
 
   i.e. value is a 2-tuple, first element of which is PTR flag ORed
-  with offset, and second is scalar element type.
+  with offset, and second is a scalar element type.
 
 * Pointer to an aggregate type::
 
     "ptr2": (offset | uctypes.PTR, {"b": 0 | uctypes.UINT8}),
 
   i.e. value is a 2-tuple, first element of which is PTR flag ORed
-  with offset, second is descriptor of type pointed to.
+  with offset, second is a descriptor of type pointed to.
 
 * Bitfields::
 
     "bitf0": offset | uctypes.BFUINT16 | lsbit << uctypes.BF_POS | bitsize << uctypes.BF_LEN,
 
-  i.e. value is type of scalar value containing given bitfield (typenames are
-  similar to scalar types, but prefixes with "BF"), ORed with offset for
+  i.e. value is a type of scalar value containing given bitfield (typenames are
+  similar to scalar types, but prefixes with ``BF``), ORed with offset for
   scalar value containing the bitfield, and further ORed with values for
-  bit offset and bit length of the bitfield within scalar value, shifted by
-  BF_POS and BF_LEN positions, respectively. Bitfield position is counted
-  from the least significant bit, and is the number of right-most bit of a
-  field (in other words, it's a number of bits a scalar needs to be shifted
-  right to extract the bitfield).
+  bit position and bit length of the bitfield within the scalar value, shifted by
+  BF_POS and BF_LEN bits, respectively. A bitfield position is counted
+  from the least significant bit of the scalar (having position of 0), and
+  is the number of right-most bit of a field (in other words, it's a number
+  of bits a scalar needs to be shifted right to extract the bitfield).
 
   In the example above, first a UINT16 value will be extracted at offset 0
   (this detail may be important when accessing hardware registers, where
@@ -126,10 +200,11 @@ Module contents
    Layout type for a native structure - with data endianness and alignment
    conforming to the ABI of the system on which MicroPython runs.
 
-.. function:: sizeof(struct)
+.. function:: sizeof(struct, layout_type=NATIVE)
 
-   Return size of data structure in bytes. Argument can be either structure
-   class or specific instantiated structure object (or its aggregate field).
+   Return size of data structure in bytes. The *struct* argument can be
+   either a structure class or a specific instantiated structure object
+   (or its aggregate field).
 
 .. function:: addressof(obj)
 
@@ -151,6 +226,35 @@ Module contents
    so it can be both written too, and you will access current value
    at the given memory address.
 
+.. data:: UINT8
+          INT8
+          UINT16
+          INT16
+          UINT32
+          INT32
+          UINT64
+          INT64
+
+   Integer types for structure descriptors. Constants for 8, 16, 32,
+   and 64 bit types are provided, both signed and unsigned.
+
+.. data:: FLOAT32
+          FLOAT64
+
+   Floating-point types for structure descriptors.
+
+.. data:: VOID
+
+   ``VOID`` is an alias for ``UINT8``, and is provided to conviniently define
+   C's void pointers: ``(uctypes.PTR, uctypes.VOID)``.
+
+.. data:: PTR
+          ARRAY
+
+   Type constants for pointers and arrays. Note that there is no explicit
+   constant for structures, it's implicit: an aggregate type without ``PTR``
+   or ``ARRAY`` flags is a structure.
+
 Structure descriptors and instantiating structure objects
 ---------------------------------------------------------
 
@@ -163,7 +267,7 @@ following sources:
   system. Lookup these addresses in datasheet for a particular MCU/SoC.
 * As a return value from a call to some FFI (Foreign Function Interface)
   function.
-* From uctypes.addressof(), when you want to pass arguments to an FFI
+* From `uctypes.addressof()`, when you want to pass arguments to an FFI
   function, or alternatively, to access some data for I/O (for example,
   data read from a file or network socket).
 
@@ -181,30 +285,41 @@ the standard subscript operator ``[]`` - both read and assigned to.
 
 If a field is a pointer, it can be dereferenced using ``[0]`` syntax
 (corresponding to C ``*`` operator, though ``[0]`` works in C too).
-Subscripting a pointer with other integer values but 0 are supported too,
+Subscripting a pointer with other integer values but 0 are also supported,
 with the same semantics as in C.
 
-Summing up, accessing structure fields generally follows C syntax,
+Summing up, accessing structure fields generally follows the C syntax,
 except for pointer dereference, when you need to use ``[0]`` operator
 instead of ``*``.
 
 Limitations
 -----------
 
-Accessing non-scalar fields leads to allocation of intermediate objects
+1. Accessing non-scalar fields leads to allocation of intermediate objects
 to represent them. This means that special care should be taken to
 layout a structure which needs to be accessed when memory allocation
 is disabled (e.g. from an interrupt). The recommendations are:
 
-* Avoid nested structures. For example, instead of
+* Avoid accessing nested structures. For example, instead of
   ``mcu_registers.peripheral_a.register1``, define separate layout
   descriptors for each peripheral, to be accessed as
-  ``peripheral_a.register1``.
-* Avoid other non-scalar data, like array. For example, instead of
-  ``peripheral_a.register[0]`` use ``peripheral_a.register0``.
-
-Note that these recommendations will lead to decreased readability
-and conciseness of layouts, so they should be used only if the need
-to access structure fields without allocation is anticipated (it's
-even possible to define 2 parallel layouts - one for normal usage,
-and a restricted one to use when memory allocation is prohibited).
+  ``peripheral_a.register1``. Or just cache a particular peripheral:
+  ``peripheral_a = mcu_registers.peripheral_a``. If a register
+  consists of multiple bitfields, you would need to cache references
+  to a particular register: ``reg_a = mcu_registers.peripheral_a.reg_a``.
+* Avoid other non-scalar data, like arrays. For example, instead of
+  ``peripheral_a.register[0]`` use ``peripheral_a.register0``. Again,
+  an alternative is to cache intermediate values, e.g.
+  ``register0 = peripheral_a.register[0]``.
+
+2. Range of offsets supported by the ``uctypes`` module is limited.
+The exact range supported is considered an implementation detail,
+and the general suggestion is to split structure definitions to
+cover from a few kilobytes to a few dozen of kilobytes maximum.
+In most cases, this is a natural situation anyway, e.g. it doesn't make
+sense to define all registers of an MCU (spread over 32-bit address
+space) in one structure, but rather a peripheral block by peripheral
+block. In some extreme cases, you may need to split a structure in
+several parts artificially (e.g. if accessing native data structure
+with multi-megabyte array in the middle, though that would be a very
+synthetic case).