MIPS: Loongson-3: Enable Store Fill Buffer at runtime
New Loongson-3 (Loongson-3A R2, Loongson-3A R3, and newer) has SFB
(Store Fill Buffer) which can improve the performance of memory access.
Now, SFB enablement is controlled by CONFIG_LOONGSON3_ENHANCEMENT, and
the generic kernel has no benefit from SFB (even it is running on a new
Loongson-3 machine). With this patch, we can enable SFB at runtime by
detecting the CPU type (the expense is war_io_reorder_wmb() will always
be a 'sync', which will hurt the performance of old Loongson-3).
[paul.burton@mips.com: Further info from Huacai:
In practise, I found that sometimes there are boot failures if I
enable SFB/LPA in cpu_probe(). I don't know why because processor
designers also haven't give me an explaination, but I think this may
have some relationships to speculative execution.]