CONFIG_HIGHMEM or CONFIG_HIGHMEM64G in illegal_highdma()

[Marin Mitov]

Hi all,

The usage of NETIF_F_HIGHDMA in net drivers make me think its 
purpose is to declare drivers's capability to address RAM above 4GB, OK?

For x86_64 CONFIG_HIGHMEM is not defined, so the function 
illegal_highdma() found in net/core/dev.c returns 0 because 
x86_64 has IOMMU mapping all RAM.

For x86_32 CONFIG_HIGHMEM is defined as 
(CONFIG_HIGHMEM4G || CONFIG_HIGHMEM64G)
While CONFIG_HIGHMEM4G insures RAM is bellow 4GB some checks
are unnecessarily done. 

Should we have CONFIG_HIGHMEM64G in illegal_highdma()
instead of CONFIG_HIGHMEM?

Regards.

Marin Mitov
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