How to reduce 1 Gigabyte MT41K128M16 DDR3 RAM to 512 Mega Bytes in mx6sabresd

[wei_li]

I  am trying to figure out  how to reduce 1 Gigabyte DDR3 RAM to 512Mega Bytes (half of 1 Gigabyte) in  mx6 sabresd  code of '/u-boot-  fslc/v2016.07+gitAUTOINC+e6b42411ab-r0 ' and hardware design. Please see my 'Question X'.

The original reference design of i.MX6 had a total of 1 Gigabyte DDR3 RAM. The implementation was using four Micron MT41K128M16 parts. Each part is a 256 Mega Bytes ( 256 M * 4 = 1Gbytes).

Question 1:  Is there any hardware change for reducing 1 Gigabyte DDR3 RAM to 512MegaByte?

My hardware design follows the original reference design for 1 Gigabyte DDR3 RAM, but there are only two MT41K128M16 parts soldered on a board. That means that there are only 512 Mega bytes (256M * 2 = 512 M) physical ram parts. 

To extend RAM size, there should be hardware changes. But  in my case, I am trying to reduce RAM. Therefore, 1 Gigabyte RAM hardware should accommodate 512Mbyte RAM without any modification. Correct?

Question 2:  Let's assume that there is no change in hardware which is same to 1Gbyte RAM original reference design,

what should be changed in u-boot software?

Here are what MDCTL and MDMISC  are shown in (see attached file)      "MX6DL_SabreSD_DDR3_register_programming_aid_v2.1.xlsx "

Reg name      Reg address        Reg value

MDCTL           0x021B0000        0x831A0000

MDMISC          0x021B0018        0x00001740

Here is what  MDCTL and MDMISC  are shown in

"/home/sw/fsl-community-bsp/build/tmp/work/imx6dlsabresd-poky-linux-gnueabi/u-boot-  fslc/v2016.07+gitAUTOINC+e6b42411ab-r0/git/board/freescale/mx6sabresd/mx6q_4x_mt41j128.cfg".

DATA 4 0x021b0000 0x831A0000

DATA 4 0x021b0018 0x00081740 (Question 3, why this value is not 0x00001740? )

Here is what  MDCTL and MDMISC  are shown in

"/home/sw/fsl-community-bsp/build/tmp/work/imx6dlsabresd-poky-linux-gnueabi/u-boot-fslc/v2016.07+gitAUTOINC+e6b42411ab-r0/git/board/freescale/mx6sabresd/mx6sabresd.cfg".

DATA 4 0x021b0000 0x831A0000

DATA 4 0x021b0018 0x00001740

Here is how 1 Gbytes Ram has been calculated in

"/home/sw/fsl-community-bsp/build/tmp/work/imx6dlsabresd-poky-linux-gnueabi/u-boot-fslc/v2016.07+gitAUTOINC+e6b42411ab-r0/git/arch/arm/imx-common/cpu.c"

---------------------------code in cpu.c ---------------------------------------------------------

#if defined(CONFIG_MX53) || defined(CONFIG_MX6)
#if defined(CONFIG_MX53)
#define MEMCTL_BASE    ESDCTL_BASE_ADDR
#else
#define MEMCTL_BASE    MMDC_P0_BASE_ADDR      /* Question 4:  MMDC_P0_BASE_ADDR must be 0x021b0000, correct? */  
#endif
static const unsigned char col_lookup[] = {9, 10, 11, 8, 12, 9, 9, 9}; /*Question 5:  how do I know these values are correct? */
static const unsigned char bank_lookup[] = {3, 2}; /*Question 6:  how are these values associated with ram bank? */

/* these MMDC registers are common to the IMX53 and IMX6 */
struct esd_mmdc_regs {
    uint32_t    ctl;
    uint32_t    pdc;
    uint32_t    otc;
    uint32_t    cfg0;
    uint32_t    cfg1;
    uint32_t    cfg2;
    uint32_t    misc;
};

#define ESD_MMDC_CTL_GET_ROW(mdctl)    ((ctl >> 24) & 7)
#define ESD_MMDC_CTL_GET_COLUMN(mdctl)    ((ctl >> 20) & 7)
#define ESD_MMDC_CTL_GET_WIDTH(mdctl)    ((ctl >> 16) & 3)
#define ESD_MMDC_CTL_GET_CS1(mdctl)    ((ctl >> 30) & 1)
#define ESD_MMDC_MISC_GET_BANK(mdmisc)    ((misc >> 5) & 1) /* Question 7: it seems that  MDMISC register needs a new value to reduce Ram from 1 Gbytes to 512 bytes. What is the exact value ? For 1Gytes value, it is 0x00001740 */ 

/*
 * imx_ddr_size - return size in bytes of DRAM according MMDC config
 * The MMDC MDCTL register holds the number of bits for row, col, and data
 * width and the MMDC MDMISC register holds the number of banks. Combine
 * all these bits to determine the meme size the MMDC has been configured for
 */
unsigned imx_ddr_size(void)
{

    struct esd_mmdc_regs *mem = (struct esd_mmdc_regs *)MEMCTL_BASE;
    unsigned ctl = readl(&mem->ctl);
    unsigned misc = readl(&mem->misc);
    int bits = 11 + 0 + 0 + 1;      /* row + col + bank + width */ /* Question 8: where do 11, 0, 0, 1 values come ? */  

    bits += ESD_MMDC_CTL_GET_ROW(ctl);
    bits += col_lookup[ESD_MMDC_CTL_GET_COLUMN(ctl)];
    bits += bank_lookup[ESD_MMDC_MISC_GET_BANK(misc)];
    bits += ESD_MMDC_CTL_GET_WIDTH(ctl);
    bits += ESD_MMDC_CTL_GET_CS1(ctl);

    /* The MX6 can do only 3840 MiB of DRAM */
    if (bits == 32)
        return 0xf0000000;

    return 1 << bits;
}
#endif

-----------------------end of code in cpu.c -----------------------------------------------------

Thanks.