Add README file for ECC
Describe more details of ECC.
Change-Id: Id5af45573bab670196b475c2c7df54f9057cace8
Signed-off-by: Will Liang <will.liang@quantatw.com>
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+# phosphor-ecc
+
+### Overview
+
+This function is to provide the BMC with the ability to record ECC error SELs.
+The EDAC driver of BMC can detects and corrects memory errors, which helps
+identify problems before they become catastrophic faulty memory module.
+
+### Requirements
+
+* The EDAC driver must be supported and enabled
+* The `phosphor-sel-logger` package must be installed
+
+### Monitor Daemon
+
+Run the application and look up the ECC error count every second after service
+is started. On first start, it resets all correctable ECC counts and
+uncorrectable ECC counts in the EDAC driver.
+
+it also provide the following path on D-Bus:
+
+* bus name : `xyz.openbmc_project.Memory.ECC`
+* object path : `/xyz/openbmc_project/metrics/memory/BmcECC`
+* interface : `xyz.openbmc_project.Memory.MemoryECC`
+
+The interface with the following properties:
+
+| Property | Type | Description |
+| -------- | ---- | ----------- |
+| isLoggingLimitReached | bool | ECC logging reach limits|
+| ceCount| int64 | correctable ECC events |
+| ueCount| int64 | uncorrectable ECC events |
+| state| string | bmc ECC event state |
+
+It also devised a mechanism to limit the "maximum number" of logs to avoid
+creating a large number of correctable ECC logs. When the `maximum quantity` is
+reached, the ECC service will stop to record the ECC log. The `maximum quantity`
+(default:100) is saved in the configuration file, and the user can modify the
+value if necessary.
+
+### Create the ECC SEL
+
+Use the `phosphor-sel-logger` package to record the following logs in BMC SEL
+format.
+
+* correctable ECC log : when fetching the `ce_count` from EDAC driver parameter
+ and the count exceeds previous count.
+* uncorrectable ECC log : when fetching the `ue_count` from EDAC driver
+ parameter and the count exceeds previous count.
+* logging limit reached log : When the correctable ECC log reaches the
+ `maximum quantity`.
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