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amd/amdmct/mct_ddr3: Improve SPD DIMM detect reliability 

Upon bootup the hardware reads at minimum 256 * 16 bytes (4Kb, or 32KB) over
I2C on a system with all DIMM slots populated.  If even one of those reads
has a single flipped bit in it (due to EMI, cross coupling with another trace,
 or just poor margins on some cheap DIMM) the system will hang and require a
hard reset.  In practice I've seen failure rates as high as 1 failed boot in
50 due to this issue, granted with cheap DIMMs, but even so retrying the read
resolves the corruption issue.

I2C is not designed for continuous data transmission with high reliability, and
there is no hardware error checking, therefore a single retry when transferring
this amount of data makes sense.

Change-Id: Ifab63eca2233c63a6a42ab8b7e742f8e47fb2a09
Signed-off-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-on: http://review.coreboot.org/11975
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
This commit is contained in:
Timothy Pearson 2015-06-04 00:10:03 -05:00 committed by Stefan Reinauer
parent 7593bda5a8
commit 45ded7df03
1 changed files with 12 additions and 1 deletions
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13
src/northbridge/amd/amdmct/mct_ddr3/mct_d.c
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@ -3642,6 +3642,7 @@ static u8 DIMMPresence_D(struct MCTStatStruc *pMCTstat,
u8 devwidth;
u16 DimmSlots;
u8 byte = 0, bytex;
uint8_t crc_status;
/* preload data structure with addrs */
mctGet_DIMMAddr(pDCTstat, pDCTstat->Node_ID);
@ -3662,10 +3663,20 @@ static u8 DIMMPresence_D(struct MCTStatStruc *pMCTstat,
int status;
smbaddr = Get_DIMMAddress_D(pDCTstat, i);
status = mctRead_SPD(smbaddr, SPD_ByteUse);
if (status >= 0) {
/* Verify result */
status = mctRead_SPD(smbaddr, SPD_ByteUse);
}
if (status >= 0) { /* SPD access is ok */
pDCTstat->DIMMPresent |= 1 << i;
read_spd_bytes(pMCTstat, pDCTstat, i);
if (crcCheck(pDCTstat, i)) { /* CRC is OK */
crc_status = crcCheck(pDCTstat, i);
if (!crc_status) {
/* Try again in case there was a transient glitch */
read_spd_bytes(pMCTstat, pDCTstat, i);
crc_status = crcCheck(pDCTstat, i);
}
if (crc_status) { /* CRC is OK */
byte = pDCTstat->spd_data.spd_bytes[i][SPD_TYPE];
if (byte == JED_DDR3SDRAM) {
/*Dimm is 'Present'*/