[Skiboot] [PATCH] tpm_i2c_nuvoton: check TPM vendor id register during probe

[Oliver O'Halloran]

On Fri, Mar 6, 2020 at 5:55 AM Eric Richter <erichte at linux.ibm.com> wrote:
>
> The driver for the nuvoton i2c TPM does not currently check if there is
> a functional TPM at the bus and address given by the device tree.
>
> This patch adds a simple check of the TPM vendor id register, compares
> against the known expected value for the chip, skips registering it if
> the chip is inaccessible or returns an unexpected id.
>
> Signed-off-by: Eric Richter <erichte at linux.ibm.com>
> ---
>  libstb/drivers/tpm_i2c_nuvoton.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/libstb/drivers/tpm_i2c_nuvoton.c b/libstb/drivers/tpm_i2c_nuvoton.c
> index 3679ddaf..882018e4 100644
> --- a/libstb/drivers/tpm_i2c_nuvoton.c
> +++ b/libstb/drivers/tpm_i2c_nuvoton.c
> @@ -30,6 +30,7 @@
>  #define TPM_BURST_COUNT                0x01
>  #define TPM_DATA_FIFO_W                0x20
>  #define TPM_DATA_FIFO_R                0x40
> +#define TPM_VID_DID            0x60
>
>  /* Bit masks for the TPM STATUS register */
>  #define TPM_STS_VALID          0x80
> @@ -42,6 +43,8 @@
>  /* TPM Driver values */
>  #define MAX_STSVALID_POLLS     5
>  #define TPM_TIMEOUT_INTERVAL   10
> +#define TPM_650_VENDOR_ID      0x5010FE00

If only we knew the name of the vendor...

> +#define TPM_VENDOR_ID_MASK     0xFFFFFF00
>
>  static struct tpm_dev *tpm_device = NULL;
>
> @@ -552,6 +555,7 @@ void tpm_i2c_nuvoton_probe(void)
>         struct dt_node *node = NULL;
>         struct i2c_bus *bus;
>         const char *name;
> +       uint32_t vendor = 0;
>
>         dt_for_each_compatible(dt_root, node, "nuvoton,npct650") {
>                 if (!dt_node_is_enabled(node))
> @@ -588,6 +592,19 @@ void tpm_i2c_nuvoton_probe(void)
>                               "found, tpm node parent %p\n", node->parent);
>                         goto disable;
>                 }
> +               /* ensure there's really the TPM we expect at that address */
> +               if (tpm_i2c_request_send(tpm_device, SMBUS_READ, TPM_VID_DID,
> +                                        1, &vendor, sizeof(vendor)) ||
> +                   (vendor & TPM_VENDOR_ID_MASK) != TPM_650_VENDOR_ID) {
> +                       prlog(PR_ERR, "NUVOTON: i2c device inaccessible, or "
> +                             "expected vendor id mismatch\n");
> +                       if (vendor) {
> +                               prlog(PR_ERR, "NUVOTON: expected 0x%X, "
> +                                     "got 0x%X\n", TPM_650_VENDOR_ID, vendor);
> +                       }

The usual pattern for this is something like:

rc = tpm_i2c_request(...);
if (rc) {
    prlog("i2c failed");
    continue;
}

if (vendor & TPM_VENDOR_ID_MASK) != TPM_650_VENDOR_ID) {
    prlog("vendor id mismatch")
    continue;
}

The control flow is cleaner, there's less indent stacking, and there's
no multi-line if conditionals which are always a mess. The way
tpm_i2c_nuvoton_probe() is structed makes it slightly awkward since
you need to duplicate the cleanup, but it's still better IMO.

> +                       free(tpm_device);
> +                       continue;
> +               }
>                 if (tpm_register_chip(node, tpm_device,
>                                       &tpm_i2c_nuvoton_driver)) {
>                         free(tpm_device);
> --
> 2.21.1
>
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