MMIO and gcc re-ordering issue

Roland Dreier rdreier at
Wed May 28 03:38:56 EST 2008

 > Actually, this specifically should not be.  The need for mmiowb on altix
 > is because it explicitly violates some of the PCI rules that would
 > otherwise impede performance.   The compromise is that readX on altix
 > contains the needed dma flush but there's a variant operator,
 > readX_relaxed that doesn't (for drivers that know what they're doing).
 > The altix critical drivers have all been converted to use the relaxed
 > form for performance, and the unconverted ones should all operate just
 > fine (albeit potentially more slowly).

Is this a recent change?  Because as of October 2007, 76d7cc03
("IB/mthca: Use mmiowb() to avoid firmware commands getting jumbled up")
was needed.  But this was involving writel() (__raw_writel() actually,
looking at the code), not readl().  But writel_relaxed() doesn't exist
(and doesn't make sense).

 - R.

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