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Extending number of MPU regions

Posted by hdtx on February 21, 2017

I'm using GCC on a Cortex-M4F (LPC43xx family).

I need more than the 3 configurable MPU regions for this port. I decided to do away with the privileged flash area because I don't need it (privileged background map is enabled and my unprivileged flash area does not reach there, so privileged accesses are enabled by default and unprivleged accesses are blocked).

What I did:

  • added the fourth area to all my task parameters (all tasks are created using xTaskCreateRestricted);
  • took out the lines setting portMPUREGIONBASEADDRESSREG and portMPUREGIONATTRIBUTE_REG for the privileged flash area in prvSetupMPU (mpu/port.c);
  • changed a few lines at the start of mpu/portmacro.h from ~~~ #define portUNPRIVILEGEDFLASHREGION ( 0UL ) #define portPRIVILEGEDFLASHREGION ( 1UL ) #define portPRIVILEGEDRAMREGION ( 2UL ) #define portGENERALPERIPHERALSREGION ( 3UL ) #define portSTACKREGION ( 4UL ) #define portFIRSTCONFIGURABLEREGION ( 5UL ) #define portLASTCONFIGURABLEREGION ( 7UL ) ~~~ to ~~~ #define portUNPRIVILEGEDFLASHREGION ( 0UL ) #define portPRIVILEGEDRAMREGION ( 1UL ) #define portGENERALPERIPHERALSREGION ( 2UL ) #define portSTACKREGION ( 3UL ) #define portFIRSTCONFIGURABLEREGION ( 4UL ) #define portLASTCONFIGURABLEREGION ( 7UL ) ~~~

What did not work: the fourth area is never set (whatever region I put there does not get activated).

I checked portNUMCONFIGURABLEREGIONS, it's obviously 4 instead of 3 now. I looked at the FreeRTOS code and the fourth region should be working, I have no idea why it's not. I appreciate any help.

Extending number of MPU regions

Posted by hdtx on February 21, 2017

Never mind, I found it... xPortPendSVHandler in mpu/port.c only swaps in 4 regions at most (including the stack region), so I added a few lines that fixed it. If anyone has the same problem, delete line 495 and add this in its place (assuming you defined the appropriate symbol): ~~~

	"	stmia r2!, {r4-r11}					\n" // original line
	"	stmia r2, {r4-r11}					\n" // don't update r2
	"	ldmia r1!, {r4-r5}					\n" // load 1 set of MPU regs
	"	stmia r2, {r4-r5}					\n" // write 1 set of MPU regs


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