1. 19 Mar, 2010 1 commit
    • Steven Rostedt's avatar
      ring-buffer: Do 8 byte alignment for 64 bit that can not handle 4 byte align · 2271048d
      Steven Rostedt authored
      The ring buffer uses 4 byte alignment while recording events into the
      buffer, even on 64bit machines. This saves space when there are lots
      of events being recorded at 4 byte boundaries.
      
      The ring buffer has a zero copy method to write into the buffer, with
      the reserving of space and then committing it. This may cause problems
      when writing an 8 byte word into a 4 byte alignment (not 8). For x86 and
      PPC this is not an issue, but on some architectures this would cause an
      out-of-alignment exception.
      
      This patch uses CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to determine
      if it is OK to use 4 byte alignments on 64 bit machines. If it is not,
      it forces the ring buffer event header to be 8 bytes and not 4,
      and will align the length of the data to be 8 byte aligned.
      This keeps the data payload at 8 byte alignments and will allow these
      machines to run without issue.
      
      The trick to this is that the header can be either 4 bytes or 8 bytes
      depending on the length of the data payload. The 4 byte header
      has a length field that supports up to 112 bytes. If the length of
      the data is more than 112, the length field is set to zero, and the actual
      length is stored in the next 4 bytes after the header.
      
      When CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is not set, the code forces
      zero in the 4 byte header forcing the length to be stored in the 4 byte
      array, even with a small data load. It also forces the length of the
      data load to be 8 byte aligned. The combination of these two guarantee
      that the data is always at 8 byte alignment.
      Tested-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
                 (on sparc64)
      Reported-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
      Acked-by: default avatarDavid S. Miller <davem@davemloft.net>
      Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
      2271048d
  2. 15 Mar, 2010 21 commits
  3. 14 Mar, 2010 18 commits