device/mmio.h: Add bit field helpers

When accessing register with multiple bit fields, the common approach is
to use clrsetbits_le32, for example:

  clrsetbits(&reg, (1 << 0) | (0x3 << 1) | (0x7 << 10),
                   (1 << 0) | (0x1 << 1) | (0x5 << 10));

This hard to maintain because we have to calculate the mask values
manually, make sure the duplicated shift (offset) was set correctly.
And it may be even worse if the value to set will be based on some
runtime values (that many developers will do a if-block with two very
similar argument list), and leaving lots of magic numbers.

We want to encourage developers always giving field names, and have a
better way of setting fields. The proposed utility macros are:

 DEFINE_BITFIELD(name, high_bit, low_bit)
 EXTRACT_BITFIELD(value, name)
 WRITE32_BITFIELDS(addr, name, value, [name2, value2, ...])
 READ32_BITFIELD(addr, name)

Where a developer can easily convert from data sheet like

 BITS  NAME
 26:24 SEC_VIO

Into a declaration

 DEFINE_BITFIELD(SEC_VIO, 26, 24)

Then, a simple call can set the field as:

 WRITE32_BITFIELDS(&reg, SEC_VIO, 2);

That is much easier to understand than

 clrsetbits_le32(&reg, 0x7 << 24, 0x2 << 24);

And to extract the value:

 READ32_BITFIELD(&reg, SEC_VIO)

That is equivalent to:

 (read32(&reg) & 0x3) >> 24

Change-Id: I8a1b17142f7a7dc6c441b0b1ee67d60d73ec8cc8
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35463
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions