[PATCH 2/3] mmc: mmc_spi: Fix potential spec violation in receiving card response

Bin Meng
2021-02-01T04:20:33Z

From: Bin Meng <bin.meng@windriver.com>

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>
---

drivers/mmc/mmc_spi.c | 32 +++++++++++++++++---------------
1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/drivers/mmc/mmc_spi.c b/drivers/mmc/mmc_spi.c
index 85a2818..5ec6b0f 100644
--- a/drivers/mmc/mmc_spi.c
+++ b/drivers/mmc/mmc_spi.c
@@ -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;
--
2.7.4