Memory Mapped Files Using the Linux mechanism for direct access to device data

Typical file access senario Use open(), lseek(), read(), write(), close() Each involves two privilege-transitions At most 4096 bytes transferred each time Inefficient for non-sequential data access

Alternative: use ‘mmap()’ Take advantage of the paging mechanism Associate virtual addresses with the data Similar to ‘swapping’ or ‘page-cacheing’ Simple standard C programming API: –‘mmap()’ creates the memory mapping –‘munmap()’ deletes the memory mapping Example: look at ‘dump.cpp’ on website

Four easy steps 1) open the file 2) map the file 3) use the file 4) unmap the file and close the file

Device memory mapping Recall our ‘vram.c’ character driver Allowed users to access display memory But lacks efficiency for serious graphics We implement driver ‘mmap()’ method

‘mmap()’ driver-method Ideas from LDD textbook: Chapter 13 But also required lots of experimentation Four steps in the ‘mmap()’ method 1) compute map’s starting-point and length 2) check: cannot map past end-of-memory 3) mark mapped area as ‘non-swappable’ 4) request kernel to set up the page-tables

Information from vm_area_struct ‘vm_start’ is starting address in user-space ‘vm_end’ is ending address in user-space ‘vm_pgoff’ is page-offset in device memory ‘vm_page_prot’ is page-protection bitmap ‘vm_flags’ is bitmap of requested attributes ‘EAGAIN’ error-code tells kernel ‘try again’

Comparing execution-times We ccan use our ‘tsc.c’ device-driver Step 1: read and save timestamp counter Step 2: perform our drawing operations Step 3: read and save timestamp counter Step 4: subtract timestamp counter values Step 5: report the number of CPU cycles