regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C by nissampa · Pull Request #550 · sonic-net/sonic-linux-kernel

nissampa · 2026-03-27T22:13:33Z

AMD PIIX4 SMBus adapters (found on AMD SP5/EPYC platforms including Cisco 8000 series) support BYTE_DATA and WORD_DATA but lack I2C_FUNC_I2C and I2C_FUNC_SMBUS_I2C_BLOCK. For AT24_FLAG_ADDR16 EEPROMs this causes regmap_get_i2c_bus() to return -ENOTSUPP, making the EEPROM inaccessible.

Add a smbus_addr16_fallback detection and two new helpers:

at24_smbus_read(): write_byte_data sets 16-bit pointer, repeated read_byte() fetches bytes (EEPROM auto-increments)
at24_smbus_write(): write_word_data encodes address+data in one WORD

Force regmap_config.reg_bits=8 when fallback is active so regmap init succeeds; all actual I/O is handled by the SMBus helpers.

Fixes EEPROM access on Cisco 8000-series with AMD SP5 on-die PIIX4 SMBus.

mssonicbld · 2026-03-27T22:13:42Z

/azp run

azure-pipelines · 2026-03-27T22:13:51Z

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paulmenzel

Please sent the commit upstream for review.

mssonicbld · 2026-03-31T19:32:36Z

/azp run

azure-pipelines · 2026-03-31T19:32:47Z

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nissampa · 2026-03-31T21:42:10Z

Please sent the commit upstream for review.

can you let me know how to do it ?
Is there a mailing list I need to send it to ?

nissampa · 2026-04-02T22:41:22Z

@paulmenzel Can you help me in approving and merging it ?

mssonicbld · 2026-04-03T17:37:23Z

/azp run

azure-pipelines · 2026-04-03T17:37:33Z

Azure Pipelines successfully started running 1 pipeline(s).

saiarcot895 · 2026-04-04T01:46:59Z

@nissampa See https://docs.kernel.org/process/submitting-patches.html for sending patches to the appropriate mailing list. Please make sure that the email you send is in plain text (not HTML) and is formatted properly.

mssonicbld · 2026-04-04T02:16:24Z

/azp run

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nissampa · 2026-04-07T22:53:33Z

@nissampa See https://docs.kernel.org/process/submitting-patches.html for sending patches to the appropriate mailing list. Please make sure that the email you send is in plain text (not HTML) and is formatted properly.

Thank you for the instructions. I did as stated in that link

paulmenzel · 2026-04-08T08:55:31Z

Thank you for sending it upstream. For the record:

Let’s see, what the comments on v2 are.

nissampa · 2026-04-09T21:02:08Z

Thank you for sending it upstream. For the record:

v1: https://lore.kernel.org/all/20260404021920.359902-1-nissampa@cisco.com/

v2: https://lore.kernel.org/all/20260407233927.498932-1-nissampa@cisco.com/

Let’s see, what the comments on v2 are.

v2 has been accepted to 7.1

Kindly let me know what to do with this PR here.

mssonicbld · 2026-04-10T04:18:18Z

/azp run

azure-pipelines · 2026-04-10T04:18:29Z

Azure Pipelines successfully started running 1 pipeline(s).

nissampa · 2026-04-10T17:21:59Z

@bmridul

paulmenzel · 2026-04-11T07:13:17Z

Thank you for sending it upstream. For the record:

v1: https://lore.kernel.org/all/20260404021920.359902-1-nissampa@cisco.com/

v2: https://lore.kernel.org/all/20260407233927.498932-1-nissampa@cisco.com/

Let’s see, what the comments on v2 are.

v2 has been accepted to 7.1

Kindly let me know what to do with this PR here.

I can’t find the commit in https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/. The URL from the notification message is incorrect.

nissampa · 2026-04-21T22:56:27Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

nissampa · 2026-04-22T22:59:43Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

@paulmenzel please let me know on this.

nissampa · 2026-04-27T22:03:43Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

@paulmenzel please let me know on this.

@paulmenzel Kindly please let me know

paulmenzel · 2026-04-28T06:07:04Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

Please share the URL to the commit.

nissampa · 2026-04-28T17:47:34Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

Please share the URL to the commit.

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/commit/?id=bad4bd28abf4d7cb2adcb39cc0de789729d2cd69

here is my commit.

nissampa · 2026-05-06T16:26:40Z

@paulmenzel Can you please take a look at this ?

paulmenzel · 2026-05-08T14:28:29Z

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/

Can you check now ? It is there.

Please share the URL to the commit.

https://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git/commit/?id=bad4bd28abf4d7cb2adcb39cc0de789729d2cd69

here is my commit.

Awesome. It’s also in v7.1-rc1now, so is going to be in 7.1 if no errors are found.

paulmenzel

Some last minor comments. Everything looks great, and thank you for getting it upstream so quickly.

mssonicbld · 2026-05-08T21:44:41Z

/azp run

azure-pipelines · 2026-05-08T21:44:51Z

Azure Pipelines successfully started running 1 pipeline(s).

…NC_I2C Add a regmap-i2c bus type that supports 16-bit register addressing on adapters that have SMBUS_BYTE_DATA and SMBUS_WORD_DATA but lack I2C_FUNC_I2C and I2C_FUNC_SMBUS_I2C_BLOCK (e.g. AMD PIIX4 on AMD SP5/EPYC platforms, including Cisco 8000 series routers). This fixes at24 EEPROM probe failures on PIIX4: at24 3-0055: probe with driver at24 failed with error -524 The fix lives in the regmap-i2c core (per upstream review feedback from Bartosz Golaszewski and Mark Brown) so all drivers benefit from the fallback, not just at24. Signed-off-by: Nishanth Sampath Kumar <nissampa@cisco.com>

mssonicbld · 2026-05-10T00:30:38Z

/azp run

azure-pipelines · 2026-05-10T00:30:44Z

Azure Pipelines will not run the associated pipelines, because the pull request was updated after the run command was issued. Review the pull request again and issue a new run command.

nissampa · 2026-05-10T00:32:55Z

/azp run

azure-pipelines · 2026-05-10T00:33:03Z

Commenter does not have sufficient privileges for PR 550 in repo sonic-net/sonic-linux-kernel

mssonicbld · 2026-05-10T00:34:34Z

/azp run

azure-pipelines · 2026-05-10T00:34:44Z

Azure Pipelines successfully started running 1 pipeline(s).

nissampa requested a review from a team as a code owner March 27, 2026 22:13

paulmenzel suggested changes Mar 31, 2026

View reviewed changes

Comment thread patches-sonic/driver-at24-smbus-fallback-for-addr16-on-piix4.patch Outdated

Comment thread patches-sonic/driver-at24-smbus-fallback-for-addr16-on-piix4.patch Outdated

nissampa force-pushed the at24_smbus branch from 07e943c to f0a9b0b Compare March 31, 2026 19:32

nissampa requested a review from paulmenzel March 31, 2026 21:41

nissampa force-pushed the at24_smbus branch from f0a9b0b to c53bc9d Compare April 3, 2026 17:37

nissampa changed the title ~~at24: SMBus fallback read/write for 16-bit addressed EEPROMs on PIIX4~~ regmap-i2c: add SMBus byte/word reg16 bus for adapters lacking I2C_FUNC_I2C Apr 10, 2026

paulmenzel reviewed May 8, 2026

View reviewed changes

Comment thread patches-sonic/0001-regmap-i2c-add-SMBus-byte-word-reg16-bus-for-adapter.patch

Comment thread patches-sonic/cisco-regmap-i2c-smbus-byte-word-reg16.patch Outdated

Comment thread patches-sonic/0001-regmap-i2c-add-SMBus-byte-word-reg16-bus-for-adapter.patch

nissampa force-pushed the at24_smbus branch from 5fbdbff to 534c394 Compare May 8, 2026 21:44

nissampa requested a review from paulmenzel May 8, 2026 22:03

nissampa force-pushed the at24_smbus branch from 534c394 to 56b0326 Compare May 10, 2026 00:30

Merge branch 'sonic-net:master' into at24_smbus

4497c63

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