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

Jaehoon Chung
2021-02-01T21:58:51Z

> 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>

Best Regards,
Jaehoon Chung

> ---
>
> 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;
>