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mmc: mmc_spi: Fix potential spec violation in receiving card response

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After command is sent and before card response shows up on the line,
there is a variable number of clock cycles in between called Ncr.
The spec [1] says the minimum is 1 byte and the maximum is 8 bytes.

Current logic in mmc_spi_sendcmd() has a flaw that it could only work
with certain SD cards with their Ncr being just 1 byte.

When resp_match is false, the codes try to receive only 1 byte from
the SD card. On the other hand when resp_match is true, the logic
happens to be no problem as it loops until timeout to receive as many
bytes as possible to see a match of the expected resp_match_value.
However not every call to mmc_spi_sendcmd() is made with resp_match
being true hence this exposes a potential issue with SD cards that
have a larger Ncr value.

Given no issue was reported as of today, we can reasonably conclude
that all cards being used on the supported boards happen to have a 1
byte Ncr timing requirement. But a broken case can be triggered by
utilizing QEMU to emulate a larger value of Ncr (by default 1 byte
Ncr is used on QEMU). This commit fixes such potential spec violation
to improve the card compatibility.

[1] "Physical Layer Specification Version 8.00"
     chapter 7.5.1: Command / Response
     chapter 7.5.4: Timing Values

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
This commit is contained in:
Bin Meng 2021-02-02 10:48:47 +08:00 committed by Peng Fan
parent 781aad0de9
commit 2f22cb40e5
1 changed files with 17 additions and 15 deletions
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32
drivers/mmc/mmc_spi.c
View File

@ -103,29 +103,31 @@ static int mmc_spi_sendcmd(struct udevice *dev,
debug("%s: cmd%d", __func__, cmdidx);
if (resp_match) {
if (resp_match)
r = ~resp_match_value;
i = CMD_TIMEOUT;
while (i) {
ret = dm_spi_xfer(dev, 1 * 8, NULL, &r, 0);
if (ret)
return ret;
debug(" resp%d=0x%x", rpos, r);
rpos++;
i--;
i = CMD_TIMEOUT;
while (i) {
ret = dm_spi_xfer(dev, 1 * 8, NULL, &r, 0);
if (ret)
return ret;
debug(" resp%d=0x%x", rpos, r);
rpos++;
i--;
if (resp_match) {
if (r == resp_match_value)
break;
} else {
if (!(r & 0x80))
break;
}
if (!i && (r != resp_match_value))
if (!i)
return -ETIMEDOUT;
}
for (i = 0; i < resp_size; i++) {
if (i == 0 && resp_match) {
resp[i] = resp_match_value;
continue;
}
resp[0] = r;
for (i = 1; i < resp_size; i++) {
ret = dm_spi_xfer(dev, 1 * 8, NULL, &r, 0);
if (ret)
return ret;