tpm_tis_spi: Release chip select when flow control fails

The failure paths in tpm_tis_spi_transfer() do not deactivate
chip select. Send an empty message (cs_select == 0) to overcome
this.

The patch is tested by two ways.
One way needs to touch hardware:
   1. force pull MISO pin down to GND, it emulates a forever
      'WAIT' timing.
   2. probe cs pin by an oscilloscope.
   3. load tpm_tis_spi.ko.
After loading, dmesg prints:
    "probe of spi0.0 failed with error -110"
and oscilloscope shows cs pin goes high(deactivated) after
the failure. Before the patch, cs pin keeps low.

Second way is by writing a fake spi controller.
   1. implement .transfer_one method, fill all rx buf with 0.
   2. implement .set_cs method, print the state of cs pin.
   we can see cs goes high after the failure.

Signed-off-by: Peijie Shao <shaopeijie@cestc.cn>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

Author: Peijie Shao

drivers/char/tpm/tpm_tis_spi_main.c

@@ -136,6 +136,14 @@ int tpm_tis_spi_transfer(struct tpm_tis_data *data, u32 addr, u16 len,
 	}
 
 exit:
+	if (ret < 0) {
+		/* Deactivate chip select */
+		memset(&spi_xfer, 0, sizeof(spi_xfer));
+		spi_message_init(&m);
+		spi_message_add_tail(&spi_xfer, &m);
+		spi_sync_locked(phy->spi_device, &m);
+	}
+
 	spi_bus_unlock(phy->spi_device->master);
 	return ret;
 }