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

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>

diff --git a/drivers/mmc/mmc_spi.c b/drivers/mmc/mmc_spi.c
index a06862aa48eec6f6f52f0797a4ea24468210518e..fbdbcf73ff4dc9d126a1ae9c47130000a4d69137 100644 (file)
--- 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;