On 10/17/2017 03:16 PM, Rob Herring wrote:
On Fri, Oct 13, 2017 at 01:35:27PM -0700, Guenter Roeck wrote:
On Fri, Oct 13, 2017 at 04:26:57PM +0200, Peter Rosin wrote:
On 2017-10-13 15:50, Guenter Roeck wrote:
On 10/13/2017 02:27 AM, Peter Rosin wrote:
With a nxp,se97 chip on an atmel sama5d31 board, the I2C adapter driver
is not always capable of avoiding the 25-35 ms timeout as specified by
the SMBUS protocol. This may cause silent corruption of the last bit of
any transfer, e.g. a one is read instead of a zero if the sensor chip
times out. This also affects the eeprom half of the nxp-se97 chip, where
this silent corruption was originally noticed. Other I2C adapters probably
suffer similar issues, e.g. bit-banging comes to mind as risky...
The SMBUS register in the nxp chip is not a standard Jedec register, but
it is not special to the nxp chips either, at least the atmel chips
have the same mechanism. Therefore, do not special case this on the
manufacturer, it is opt-in via the device property anyway.
Signed-off-by: Peter Rosin <peda@xxxxxxxxxx>
---
Documentation/devicetree/bindings/hwmon/jc42.txt | 4 ++++
drivers/hwmon/jc42.c | 20 ++++++++++++++++++++
2 files changed, 24 insertions(+)
diff --git a/Documentation/devicetree/bindings/hwmon/jc42.txt b/Documentation/devicetree/bindings/hwmon/jc42.txt
index 07a250498fbb..f569db58f64a 100644
--- a/Documentation/devicetree/bindings/hwmon/jc42.txt
+++ b/Documentation/devicetree/bindings/hwmon/jc42.txt
@@ -34,6 +34,10 @@ Required properties:
- reg: I2C address
+Optional properties:
+- smbus-timeout-disable: When set, the smbus timeout function will be disabled.
+ This is not supported on all chips.
Is this only for jc24 devices or could be any smbus device?
SMBus timeout is a standard SMBus functionality, so I would say any. It is by
default enabled on an SMBus device (actually it is not just enabled, it is
mandatory). The ability to disable it comes handy if a SMBus chip is connected
to an I2C controller which does not (or not necessarily) follow SMBus rules.
I had seen that problem myself with MAX6697, and STTS751 (and its driver) also
supports it.
+
Example:
temp-sensor@1a {
diff --git a/drivers/hwmon/jc42.c b/drivers/hwmon/jc42.c
index 1bf22eff0b08..fd816902fa30 100644
--- a/drivers/hwmon/jc42.c
+++ b/drivers/hwmon/jc42.c
@@ -45,6 +45,7 @@ static const unsigned short normal_i2c[] = {
#define JC42_REG_TEMP 0x05
#define JC42_REG_MANID 0x06
#define JC42_REG_DEVICEID 0x07
+#define JC42_REG_SMBUS 0x22 /* NXP and Atmel, possibly others? */
/* Status bits in temperature register */
#define JC42_ALARM_CRIT_BIT 15
@@ -73,6 +74,9 @@ static const unsigned short normal_i2c[] = {
#define ONS_MANID 0x1b09 /* ON Semiconductor */
#define STM_MANID 0x104a /* ST Microelectronics */
+/* SMBUS register */
+#define SMBUS_STMOUT BIT(7) /* SMBus time-out, active low */
+
/* Supported chips */
/* Analog Devices */
@@ -476,6 +480,22 @@ static int jc42_probe(struct i2c_client *client, const struct i2c_device_id *id)
data->extended = !!(cap & JC42_CAP_RANGE);
+ if (device_property_read_bool(dev, "smbus-timeout-disable")) {
+ int smbus;
+
+ /*
+ * Not all chips support this register, but from a
+ * quick read of various datasheets no chip appears
+ * incompatible with the below attempt to disable
+ * the timeout. And the whole thing is opt-in...
+ */
+ smbus = i2c_smbus_read_word_swapped(client, JC42_REG_SMBUS);
+ if (smbus < 0)
+ return smbus;
+ i2c_smbus_write_word_swapped(client, JC42_REG_SMBUS,
+ smbus | SMBUS_STMOUT);
Looking into the SE97 datasheet, the bit is only writable if the alarm bits
are not locked. Should we take this into account and unlock the alarm bits
if necessary ?
Right. And I thought about the case when the timeout was disabled before
probing but with the property not present (perhaps by someone trying things
out, like I have). Should the timeout be re-enabled in that case?
No, because the property only states that the timeout should be disabled.
It does not say that it should be _enabled_ if the property is not there.
That would require a different property. A -> B does not imply B -> A.
A not-present/0/1 property is typically used for such cases. Perhaps you
want that?
I don't want to change behavior if the property is not present. After all,
the timeout may have been disabled by the BIOS/ROMMON (especially in systems
w/o DT support). So far having the boolean flag was never a problem; as
mentioned above, the timeout is by default (and per spec) enabled on SMBus
devices. I would argue that anyone who disabled it must have done so on
purpose (including "trying out things"), and that it should not be DT
responsibility to have a flag along the line of "restore default
configuration".
Thanks,
Guenter
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