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List:       dri-devel
Subject:    [Bug 24781] New: Incorrect calculation of free dwords in ring buffer
From:       bugzilla-daemon () freedesktop ! org
Date:       2009-10-29 0:53:56
Message-ID: bug-24781-502 () http ! bugs ! freedesktop ! org/
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http://bugs.freedesktop.org/show_bug.cgi?id=24781

           Summary: Incorrect calculation of free dwords in ring buffer
           Product: DRI
           Version: XOrg CVS
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Radeon
        AssignedTo: dri-devel@lists.sourceforge.net
        ReportedBy: zajec5@gmail.com


Created an attachment (id=30782)
 --> (http://bugs.freedesktop.org/attachment.cgi?id=30782)
drm/radeon/kms: fix calculation of free dwords in ring

There are two scenarios of read/write pointers relation:

1) read lower than write
/----------------------------\
|     A B C D E F G          |
| 1 2 3 4 5 6 7 8 9 10 11 12 |
\----------------------------/
      R           W
used = w - r
free = size - used
free = size - (w - r)
free = size - w + r
free = r + size - w


2) write lower than read
/----------------------------\
| E F G           A  B  C  D |
| 1 2 3 4 5 6 7 8 9 10 11 12 |
\----------------------------/
      W           R
free = r - w


I made there ascii pictures to make it clear, as noone believed me today on IRC
we have some bug there ;)

Problem is we calculate free dwords for first situation only (always). We have
to detect second situation and use other calculation then.


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