Spare MPU Region

[ofirmsh]

Hello,

We are using the MIMXRT1176, equipped with 64 MB external flash running in XIP, and 64 MB SDRAM.

We'd like to make the last 16 MB of the SDRAM non-cacheable. However, looking at the default MPU configuration in the SDK, all of the regions on the M7 core (0-15) are taken (snippet below). We looked around the reference manual at these memory regions, and most of them are reserved/in use.

Are there any regions that we can safely give up on (or any other ideas) to make room for a 16 MB non-cacheable region at address 0x8300_0000?

We need to rebuild the SDRAM address space out of 3 regions total:

• 48 MB cacheable - 32 MB + 16 MB (the MPU RBAR ADDR field does not accept 48 MB, so we build it out of two regions)
• 16 MB non-cacheable

We think we can safely repurpose Region 2 (FlexSPI2, 0x6000_0000) as we are not using it (our XIP flash is on FlexSPI1), but that would mean we are still missing one more region.

Many thanks for any help!

/* MPU configuration. */
#if __CORTEX_M == 7
void BOARD_ConfigMPU(void)
{
    /* ... some variable definitions ... */

#if defined(__ICACHE_PRESENT) && __ICACHE_PRESENT
    /* Disable I cache and D cache */
    if (SCB_CCR_IC_Msk == (SCB_CCR_IC_Msk & SCB->CCR))
    {
        SCB_DisableICache();
    }
#endif
#if defined(__DCACHE_PRESENT) && __DCACHE_PRESENT
    if (SCB_CCR_DC_Msk == (SCB_CCR_DC_Msk & SCB->CCR))
    {
        SCB_DisableDCache();
    }
#endif

    /* Disable MPU */
    ARM_MPU_Disable();

    /* MPU configure:
     * Use ARM_MPU_RASR(DisableExec, AccessPermission, TypeExtField, IsShareable, IsCacheable, IsBufferable,
     * SubRegionDisable, Size)
     * API in mpu_armv7.h.
     * param DisableExec       Instruction access (XN) disable bit,0=instruction fetches enabled, 1=instruction fetches
     * disabled.
     * param AccessPermission  Data access permissions, allows you to configure read/write access for User and
     * Privileged mode.
     *      Use MACROS defined in mpu_armv7.h:
     * ARM_MPU_AP_NONE/ARM_MPU_AP_PRIV/ARM_MPU_AP_URO/ARM_MPU_AP_FULL/ARM_MPU_AP_PRO/ARM_MPU_AP_RO
     * Combine TypeExtField/IsShareable/IsCacheable/IsBufferable to configure MPU memory access attributes.
     *  TypeExtField  IsShareable  IsCacheable  IsBufferable   Memory Attribute    Shareability        Cache
     *     0             x           0           0             Strongly Ordered    shareable
     *     0             x           0           1              Device             shareable
     *     0             0           1           0              Normal             not shareable   Outer and inner write
     * through no write allocate
     *     0             0           1           1              Normal             not shareable   Outer and inner write
     * back no write allocate
     *     0             1           1           0              Normal             shareable       Outer and inner write
     * through no write allocate
     *     0             1           1           1              Normal             shareable       Outer and inner write
     * back no write allocate
     *     1             0           0           0              Normal             not shareable   outer and inner
     * noncache
     *     1             1           0           0              Normal             shareable       outer and inner
     * noncache
     *     1             0           1           1              Normal             not shareable   outer and inner write
     * back write/read acllocate
     *     1             1           1           1              Normal             shareable       outer and inner write
     * back write/read acllocate
     *     2             x           0           0              Device              not shareable
     *  Above are normal use settings, if your want to see more details or want to config different inner/outter cache
     * policy.
     *  please refer to Table 4-55 /4-56 in arm cortex-M7 generic user guide <dui0646b_cortex_m7_dgug.pdf>
     * param SubRegionDisable  Sub-region disable field. 0=sub-region is enabled, 1=sub-region is disabled.
     * param Size              Region size of the region to be configured. use ARM_MPU_REGION_SIZE_xxx MACRO in
     * mpu_armv7.h.
     */

    /*
     * Add default region to deny access to whole address space to workaround speculative prefetch.
     * Refer to Arm errata 1013783-B for more details.
     *
     */
    /* Region 0 setting: Instruction access disabled, No data access permission. */
    MPU->RBAR = ARM_MPU_RBAR(0, 0x00000000U);
    MPU->RASR = ARM_MPU_RASR(1, ARM_MPU_AP_NONE, 0, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_4GB);

    /* Region 1 setting: Memory with Device type, not shareable, non-cacheable. */
    MPU->RBAR = ARM_MPU_RBAR(1, 0x80000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_512MB);

    /* Region 2 setting: Memory with Device type, not shareable,  non-cacheable. */
    MPU->RBAR = ARM_MPU_RBAR(2, 0x60000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_512MB);

    /* Region 3 setting: Memory with Device type, not shareable, non-cacheable. */
    MPU->RBAR = ARM_MPU_RBAR(3, 0x00000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_1GB);

    /* Region 4 setting: Memory with Normal type, not shareable, outer/inner write back */
    MPU->RBAR = ARM_MPU_RBAR(4, 0x00000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_256KB);

    /* Region 5 setting: Memory with Normal type, not shareable, outer/inner write back */
    MPU->RBAR = ARM_MPU_RBAR(5, 0x20000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_256KB);

#if defined(CACHE_MODE_WRITE_THROUGH) && CACHE_MODE_WRITE_THROUGH
    /* Region 6 setting: Memory with Normal type, not shareable, write through */
    MPU->RBAR = ARM_MPU_RBAR(6, 0x20200000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 0, 0, ARM_MPU_REGION_SIZE_1MB);

    /* Region 7 setting: Memory with Normal type, not shareable, write trough */
    MPU->RBAR = ARM_MPU_RBAR(7, 0x20300000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 0, 0, ARM_MPU_REGION_SIZE_512KB);
#else
    /* Region 6 setting: Memory with Normal type, not shareable, outer/inner write back */
    MPU->RBAR = ARM_MPU_RBAR(6, 0x20200000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_1MB);

    /* Region 7 setting: Memory with Normal type, not shareable, outer/inner write back */
    MPU->RBAR = ARM_MPU_RBAR(7, 0x20300000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_512KB);
#endif

#if defined(XIP_EXTERNAL_FLASH) && (XIP_EXTERNAL_FLASH == 1)
    /* Region 8 setting: Memory with Normal type, not shareable, outer/inner write back. */
    MPU->RBAR = ARM_MPU_RBAR(8, 0x30000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_64MB);
#endif

#ifdef USE_SDRAM
#if defined(CACHE_MODE_WRITE_THROUGH) && CACHE_MODE_WRITE_THROUGH
    /* Region 9 setting: Memory with Normal type, not shareable, write trough */
    MPU->RBAR = ARM_MPU_RBAR(9, 0x80000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 0, 0, ARM_MPU_REGION_SIZE_64MB);
#else
    /* Region 9 setting: Memory with Normal type, not shareable, outer/inner write back */
    MPU->RBAR = ARM_MPU_RBAR(9, 0x80000000U);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 0, 0, 1, 1, 0, ARM_MPU_REGION_SIZE_64MB);
#endif
#endif

    while ((size >> i) > 0x1U)
    {
        i++;
    }

    if (i != 0)
    {
        /* The MPU region size should be 2^N, 5<=N<=32, region base should be multiples of size. */
        assert(!(nonCacheStart % size));
        assert(size == (uint32_t)(1 << i));
        assert(i >= 5);

        /* Region 10 setting: Memory with Normal type, not shareable, non-cacheable */
        MPU->RBAR = ARM_MPU_RBAR(10, nonCacheStart);
        MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 1, 0, 0, 0, 0, i - 1);
    }

    /* Region 11 setting: Memory with Device type, not shareable, non-cacheable */
    MPU->RBAR = ARM_MPU_RBAR(11, 0x40000000);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_16MB);

    /* Region 12 setting: Memory with Device type, not shareable, non-cacheable */
    MPU->RBAR = ARM_MPU_RBAR(12, 0x41000000);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_2MB);

    /* Region 13 setting: Memory with Device type, not shareable, non-cacheable */
    MPU->RBAR = ARM_MPU_RBAR(13, 0x41400000);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_1MB);

    /* Region 14 setting: Memory with Device type, not shareable, non-cacheable */
    MPU->RBAR = ARM_MPU_RBAR(14, 0x41800000);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_2MB);

    /* Region 15 setting: Memory with Device type, not shareable, non-cacheable */
    MPU->RBAR = ARM_MPU_RBAR(15, 0x42000000);
    MPU->RASR = ARM_MPU_RASR(0, ARM_MPU_AP_FULL, 2, 0, 0, 0, 0, ARM_MPU_REGION_SIZE_1MB);

    /* Enable MPU */
    ARM_MPU_Enable(MPU_CTRL_PRIVDEFENA_Msk);

    /* Enable I cache and D cache */
#if defined(__DCACHE_PRESENT) && __DCACHE_PRESENT
    SCB_EnableDCache();
#endif
#if defined(__ICACHE_PRESENT) && __ICACHE_PRESENT
    SCB_EnableICache();
#endif
}

[Omar_Anguiano]

In the MCU settings you can make the partition of the SDRAM so in the MPU you set the non-cache settings.
One example of this is found on some SDK examples as the EVK SDRAM has one region set to non-cache.

Best regards,
Omar

[ofirmsh]

Hi Omar,

We're using the MCUXpresso for VSCode extension, so we do not have the MCU Settings menu that is present on the MCUXpresso IDE. I will try to replicate this situation in the IDE to see how it can be done without it.

I already went through the SDRAM examples but will go over them again to see if I missed some. If you happen to know which example I should look at, do let me know.

[Omar_Anguiano · Accepted solution]

For VScode you will have to modify the linker to have other SDRAM partition. To configure the ncache region in MPU you can refer to your code attached. 

There the SDRAM region is configured as non-cache. The linker defines where this region starts. With MCUXpresso you could modify MCU settings but with VScode need to modify the linker: 

Best regards,
Omar

[ofirmsh]

Thank you, Omar, I will look into this.