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soft_i2c/i2c_probe(): Perform reads instead of writes

PATCH New Peter Korsgaard 4 participants 7 messages May 26, 2005 – Mar 12, 2006

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PATCH 1/1 soft_i2c/i2c_probe(): Perform reads instead of writes

Peter,

I believe the implementation was correct but the comment was wrong. It should be a write transaction with just address byte. After the address byte is transmitted the I2C slave will acknowledge (if present) and we finish the transaction by generating the stop condition. i.e. fake write.

Best regards,
Tolunay

Quoted 33 lines · Peter Korsgaard

Hi,

The following little patch makes the i2c_probe function use reads instead of writes to match the comment (broken in r1.2).

* Patch by Peter Korsgaard, 26 May 2005:
Use reads instead of writes for probe in soft_i2c.

------------------------------------------------------------------------

Index: common/soft_i2c.c
===================================================================
RCS file: /cvsroot/u-boot/u-boot/common/soft_i2c.c,v retrieving revision 1.5
diff -u -p -u -r1.5 soft_i2c.c

soft_i2c.c +1 -2
@@ -290,7 +290,7 @@ int i2c_probe(uchar addr)
 
 	/* perform 1 byte read transaction */
 	send_start();
-	rc = write_byte ((addr << 1) | 0);
+	rc = write_byte ((addr << 1) | 1);
 	send_stop();
 
 	return (rc ? 1 : 0);
 
 
------------------------------------------------------------------------
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#2 lore raw

PATCH 1/1 soft_i2c/i2c_probe(): Perform reads instead of writes

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"Tolunay" == Tolunay Orkun <[email protected]> writes:

Hi, and sorry for the slow response!

Tolunay> Peter, I believe the implementation was correct but the Tolunay> comment was wrong. It should be a write transaction with Tolunay> just address byte. After the address byte is transmitted the Tolunay> I2C slave will acknowledge (if present) and we finish the Tolunay> transaction by generating the stop condition. i.e. fake Tolunay> write.

Why write? Wouldn't a zero byte read transaction be safer?

Signature · 1 line
Bye, Peter Korsgaard
#3 lore raw

PATCH 1/1 soft_i2c/i2c_probe(): Perform reads instead of writes

Quoted 6 lines · Peter Korsgaard
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"Tolunay" == Tolunay Orkun <[email protected]> writes:

Hi, and sorry for the slow response!

Quoted 11 lines

Tolunay> Peter, I believe the implementation was correct but the Tolunay> comment was wrong. It should be a write transaction with Tolunay> just address byte. After the address byte is transmitted the Tolunay> I2C slave will acknowledge (if present) and we finish the Tolunay> transaction by generating the stop condition. i.e. fake Tolunay> write.

Why write? Wouldn't a zero byte read transaction be safer?

I have read somewhere (that I cannot find now), that some device would lock up on 0-byte read vs 0-byte write. Since the current code is working as it is unless there is demonstrable problem with it, I would fix the comment only.

#4 lore raw

PATCH 1/1 soft_i2c/i2c_probe(): Perform reads instead of writes

Quoted 15 lines · Tolunay Orkun
Quoted 13 lines · Peter Korsgaard
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"Tolunay" == Tolunay Orkun <[email protected]> writes:

Hi, and sorry for the slow response!

Tolunay> Peter, I believe the implementation was correct but the Tolunay> comment was wrong. It should be a write transaction with Tolunay> just address byte. After the address byte is transmitted the Tolunay> I2C slave will acknowledge (if present) and we finish the Tolunay> transaction by generating the stop condition. i.e. fake Tolunay> write.

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Why write? Wouldn't a zero byte read transaction be safer?

I have read somewhere (that I cannot find now), that some device would lock up on 0-byte read vs 0-byte write. Since the current code is working as it is unless there is demonstrable problem with it, I would fix the comment only.

Ebony (PPC440GP eval board) has a device like this, IIRC it's a clock driver, zero-read locks the bus completely.

In general, zero write is safer, also SMBUS spec defines such transaction as SMBUS QUICK; interestingly, there is no read equivalent :).

Also, from personal testing, using zero-write as detection hack, yields more devices than zero-read.

Signature · 1 line
Eugene
#6 lore raw

PATCH 1/1 soft_i2c/i2c_probe(): Perform reads instead of writes

In reply to Peter Korsgaard
Quoted 3 lines · In message <[email protected]> you wrote:

* Patch by Peter Korsgaard, 26 May 2005:
Use reads instead of writes for probe in soft_i2c.

Rejected. Tolunay's explanation was correct: only the comment was wrong. Fixed the comment now.

Best regards,

Wolfgang Denk

Signature · 6 lines
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
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Idols]_ ``Maxims and Missiles'' no. 26