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ioremap_wc on arm64

Will Deacon linux-arm-kernel 1 version New

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  1. v1 Latest0 of 2 patches 0 repliesNew

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Cover letter [PATCH 0/2] ioremap_wc on arm64
Quoted 14 lines · Jayachandran C
Quoted 6 lines · Catalin Marinas
Quoted 5 lines · Jayachandran C

From its definition, the device "gather" attribute seems to be a better fit for implementing write combining mapping in ioremap_wc(). And on ThunderX2, Device GRE mapping has optimizations that makes it much faster than normal uncached mapping.

Quoted 6 lines
Quoted 3 lines

I am not sure of the reasoning behind the original decision to make ioremap_wc use "Normal Non-Cached" attribute, since all the other variants of ioremap use device attributes, and ioremap_wc looks like an exception.

The reason we kept it as Normal NC is that Device_GRE does not allow unaligned accesses.

Quoted 7 lines

There does not to be an expectation to have unaligned access on __iomem pointers. I also see that memremap can call ioremap_wc (which is a Normal mapping) or ioremap_wt(which is a Device mapping), so that is inconsistent as well.

Was this added for a specific use case? Also, do you think this patchset is acceptable?

I think it was used for framebuffers. Note that using normal-NC also aligns with arch/arm/ and, since normal-NC is strictly a more relaxed memory type than Device-GRE, I'm really not keen making on this change.

Will

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