docs: ethtool: clarify the bit-by-bit bitset format description

Clarify the bit-by-bit bitset format's behavior around mandatory
attributes and bit identification. More specifically, the following
changes are made:

* Rephrase a misleading sentence which implies name and index are
  mutually exclusive
* Describe that ETHTOOL_A_BITSET_BITS nest is mandatory
* Describe that a request fails if inconsistent identifiers are given

Signed-off-by: Yohei Kojima <yk@y-koj.net>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/ef90a56965ca66e57aa177929ce3e10c5ca815fa.1772031974.git.yk@y-koj.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/Documentation/networking/ethtool-netlink.rst b/Documentation/networking/ethtool-netlink.rst
index af56c304cef4..32179168eb73 100644
--- a/Documentation/networking/ethtool-netlink.rst
+++ b/Documentation/networking/ethtool-netlink.rst
@@ -96,7 +96,7 @@ For short bitmaps of (reasonably) fixed length, standard ``NLA_BITFIELD32``
 type is used. For arbitrary length bitmaps, ethtool netlink uses a nested
 attribute with contents of one of two forms: compact (two binary bitmaps
 representing bit values and mask of affected bits) and bit-by-bit (list of
-bits identified by either index or name).
+bits identified by index or name).
 
 Verbose (bit-by-bit) bitsets allow sending symbolic names for bits together
 with their values which saves a round trip (when the bitset is passed in a
@@ -156,12 +156,16 @@ Bit-by-bit form: nested (bitset) attribute contents:
  | | | ``ETHTOOL_A_BITSET_BIT_VALUE`` | flag   | present if bit is set       |
  +-+-+--------------------------------+--------+-----------------------------+
 
-Bit size is optional for bit-by-bit form. ``ETHTOOL_A_BITSET_BITS`` nest can
+For bit-by-bit form, ``ETHTOOL_A_BITSET_SIZE`` is optional, and
+``ETHTOOL_A_BITSET_BITS`` is mandatory. ``ETHTOOL_A_BITSET_BITS`` nest can
 only contain ``ETHTOOL_A_BITSET_BITS_BIT`` attributes but there can be an
 arbitrary number of them.  A bit may be identified by its index or by its
 name. When used in requests, listed bits are set to 0 or 1 according to
-``ETHTOOL_A_BITSET_BIT_VALUE``, the rest is preserved. A request fails if
-index exceeds kernel bit length or if name is not recognized.
+``ETHTOOL_A_BITSET_BIT_VALUE``, the rest is preserved.
+
+A request fails if index exceeds kernel bit length or if name is not
+recognized. If both name and index are set, the request will fail if they
+point to different bits.
 
 When ``ETHTOOL_A_BITSET_NOMASK`` flag is present, bitset is interpreted as
 a simple bitmap. ``ETHTOOL_A_BITSET_BIT_VALUE`` attributes are not used in