nvmem: core: Add functions to make number reading easy

nvmem: core: Add functions to make number reading easy

Sometimes the clients of nvmem just want to get a number out of
nvmem. They don't want to think about exactly how many bytes the nvmem
cell took up. They just want the number. Let's make it easy.

In general this concept is useful because nvmem space is precious and
usually the fewest bits are allocated that will hold a given value on
a given system. However, even though small numbers might be fine on
one system that doesn't mean that logically the number couldn't be
bigger. Imagine nvmem containing a max frequency for a component. On
one system perhaps that fits in 16 bits. On another system it might
fit in 32 bits. The code reading this number doesn't care--it just
wants the number.

We'll provide two functions: nvmem_cell_read_variable_le_u32() and
nvmem_cell_read_variable_le_u64().

Comparing these to the existing functions like nvmem_cell_read_u32():
* These new functions have no problems if the value was stored in
  nvmem in fewer bytes. It's OK to use these function as long as the
  value stored will fit in 32-bits (or 64-bits).
* These functions avoid problems that the earlier APIs had with bit
  offsets. For instance, you can't use nvmem_cell_read_u32() to read a
  value has nbits=32 and bit_offset=4 because the nvmem cell must be
  at least 5 bytes big to hold this value. The new API accounts for
  this and works fine.
* These functions make it very explicit that they assume that the
  number was stored in little endian format. The old functions made
  this assumption whenever bit_offset was non-zero (see
  nvmem_shift_read_buffer_in_place()) but didn't whenever the
  bit_offset was zero.

NOTE: it's assumed that we don't need an 8-bit or 16-bit version of
this function. The 32-bit version of the function can be used to read
8-bit or 16-bit data.

At the moment, I'm only adding the "unsigned" versions of these
functions, but if it ends up being useful someone could add a "signed"
version that did 2's complement sign extension.

At the moment, I'm only adding the "little endian" versions of these
functions. Adding the "big endian" version would require adding "big
endian" support to nvmem_shift_read_buffer_in_place().

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20210330111241.19401-7-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>