[PATCH v3] tpm_tis_spi: Release chip select when flow control fails

[shaopeijie@cestc.cn]

From: Peijie Shao <shaopeijie@cestc.cn>

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>
---
Changes since v1:
    Thanks to BR, Jarkko's suggestions.
    1. Deactivate cs all of the failure path, not only flow control.
    2. change and update comments.
---
Changes since v2:
    Thanks to Paul?Menzel's suggestions.
    1. common commit tag and correct spelling mistakes.
    2. add test steps.
---
 drivers/char/tpm/tpm_tis_spi_main.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
index 1f5207974..9bfaba092 100644
--- a/drivers/char/tpm/tpm_tis_spi_main.c
+++ b/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;
 }
-- 
2.39.1