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Bug #14891

open

dma_attr_burstsizes is confusing

Added by Garrett D'Amore 8 days ago.

Status:
New
Priority:
Normal
Assignee:
-
Category:
-
Start date:
Due date:
% Done:

0%

Estimated time:
Difficulty:
Medium
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Gerrit CR:

Description

Folks coming at the system with a PCI and x86 specific background will be unfamiliar with the notion of burst size configuration. This is because this is not something one normally needs to configure for such busses.

On sbus, configuration of the burstsizes is necessary to work properly with the streaming cache, but modern system designs typically don't require this configuration.

Many of our drivers have values seemingly chosen at random -- probably a lot of them come via copy-paste from other drivers, which go all the way back to the old Sbus based designs.

In fact, only a tiny percentage of drivers use the value with the ddi_dma_burstsizes() API, which is meant to be used to configure bursting, correctly. And those are mostly derivers that have some legacy in supporting Sbus, or at least SPARC, systems.

We should go ahead and do the following:

1. Update the ddi_dma_attr(9S) man page to recommend that drivers for PCI buses or busses that do not provide for configuration of burst sizes can simply supply 1 for this value.
2. Consider making ddi_dma_burstsizes() obsolete. (The only callers it has are either SPARC specific, or do nothing useful with the value.)

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