Hi, I am seeing some strange behavior while accessing buffers through User Space (mapped using mmap call)
Background :- ------------ Platform - TI AM3517 CPU - ARM Cortex A8 r...@am3517-evm:~# r...@am3517-evm:~# cat /proc/cpuinfo Processor : ARMv7 Processor rev 7 (v7l) BogoMIPS : 499.92 Features : swp half thumb fastmult vfp edsp neon vfpv3 CPU implementer : 0x41 CPU architecture: 7 CPU variant : 0x1 CPU part : 0xc08 CPU revision : 7 Hardware : OMAP3517/AM3517 EVM Revision : 0020 Serial : 0000000000000000 r...@omap3517-evm:~# Issue/Usage :- ------------- The V4l2-Capture driver captures the data from video decoder into buffer and the application does some processing on this buffer. The mmap implementation can be found at drivers/media/video/videobuf-dma-contig.c, function__videobuf_mmap_mapper(). Observation - The CPU consumption goes as high as 95% on read buffer operation, please note that write operation on these buffers also gives 60-70% CPU consumption. (Using memcpy/memset API's for read and write operation). Some more inputs :- ------------------ - If I specify PAGE_READONLY or PAGE_SHARED (actual flag is L_PTE_USER) while mapping the buffer to UserSpace in mmap system call, the CPU consumption goes down to expected value (20-27%). Then I reached till the function cpu_v7_set_pte_ext, where we are configuring level 2 translation table entries, which makes use of these flags. - Below is the value of r0, r1 and r2 register (ptep, pteval, ext) in both the cases - Without PAGE_READONLY/PAGE_SHARED ptep - cfb5de10, pte - 8d200383, ext - 800 ptep - cfb5de14, pte - 8d201383, ext - 800 Important bits are [0-9] - 0x383 With PAGE_READONLY/PAGE_SHARED set ptep - cfb30e10, pte - 8d10038f, ext - 800 ptep - cfb30e14, pte - 8d10138f, ext - 800 Important bits are [0-9] - 0x38F The lines inside function "cpu_v7_set_pte_ext", is using the flag as shown below - tst r1, #L_PTE_USER orrne r3, r3, #PTE_EXT_AP1 tstne r3, #PTE_EXT_APX bicne r3, r3, #PTE_EXT_APX | PTE_EXT_AP0 Without PAGE_READONLY/PAGE_SHARED With flags set Access perm = reserved Access Perm = Read Only - I tried the same thing with another platform (ARM9) and it works fine there. Can somebody help me to understand the flag PAGE_SHARED/PAGE_READONLY and access permissions? Am I debugging this into right path? Does anybody have seen/observed similar issue before? Thanks in advance. Thanks, Vaibhav Hiremath -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
