Commit dd0c41f8 authored by Committed by Bartlomiej ZolnierkiewiczBrowse files
efifb: allow user to disable write combined mapping.
This patch allows the user to disable write combined mapping of the efifb framebuffer console using an nowc option. A customer noticed major slowdowns while logging to the console with write combining enabled, on other tasks running on the same CPU. (10x or greater slow down on all other cores on the same CPU as is doing the logging). I reproduced this on a machine with dual CPUs. Intel(R) Xeon(R) CPU E5-2609 v3 @ 1.90GHz (6 core) I wrote a test that just mmaps the pci bar and writes to it in a loop, while this was running in the background one a single core with (taskset -c 1), building a kernel up to init/version.o (taskset -c 8) went from 13s to 133s or so. I've yet to explain why this occurs or what is going wrong I haven't managed to find a perf command that in any way gives insight into this. 11,885,070,715 instructions # 1.39 insns per cycle vs 12,082,592,342 instructions # 0.13 insns per cycle is the only thing I've spotted of interest, I've tried at least: dTLB-stores,dTLB-store-misses,L1-dcache-stores,LLC-store,LLC-store-misses,LLC-load-misses,LLC-loads,\mem-loads,mem-stores,iTLB-loads,iTLB-load-misses,cache-references,cache-misses For now it seems at least a good idea to allow a user to disable write combining if they see this until we can figure it out. Note also most users get a real framebuffer driver loaded when kms kicks in, it just happens on these machines the kernel didn't support the gpu specific driver. Signed-off-by: Dave Airlie <email@example.com> Acked-by: Peter Jones <firstname.lastname@example.org> Cc: Andy Lutomirski <email@example.com> Cc: H. Peter Anvin <firstname.lastname@example.org> Cc: Linus Torvalds <email@example.com> Signed-off-by: Bartlomiej Zolnierkiewicz <firstname.lastname@example.org>
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