Why it's a "mask" ...

Why it's a "mask" ...

Posted Mar 20, 2008 7:29 UTC (Thu) by HalfMoon (guest, #3211)
In reply to: A better DMA memory allocator by iabervon
Parent article: A better DMA memory allocator

The classic example of that is that SA-1100 (old ARMv4, no longer manufactured) DMA controller had an erratum which meant that one address bit could never be used ... every other MByte was unusable for DMA. So this is a case where the functionality of a DMA address "mask" was appropriate, instead of just a "biggest address" value. Intel never fixed that bug (or a boatload of others in that chip).

It escapes me why Linux calls what it has a "mask"; it's long overdue to change its name to reflect the fact that it's just a ceiling on the addresses usable for DMA. Calling it a "mask" makes it seem like the complete inability to handle that SA-1100 erratum is a bug.

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