UBIFS: add new faq entry

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --git a/faq/ubifs.xml b/faq/ubifs.xml
index f29943e36beff2b8b7f042006d6e16cd2a7f8e06..e683fcd8c7f75d04896b1c8e19f1dd9a9d1dd046 100644 (file)
--- a/faq/ubifs.xml
+++ b/faq/ubifs.xml
@@ -38,6 +38,7 @@
        <li><a href="ubifs.html#L_bgt_thread">What does the "ubifs_bgt0_0" thread do?</a></li>
        <li><a href="ubifs.html#L_sudden_ro">UBIFS suddenly became read-only - what is this?</a></li>
        <li><a href="ubifs.html#L_lebsz_mismatch">I see this UBIFS error: "validate_sb: LEB size mismatch: 129024 in superblock, 126976 real"</a></li>
+       <li><a href="ubifs.html#L_ecc_error">I see this UBI error: "ubi_io_read: error -74 while reading 126976 bytes from PEB 47:4096, read 126976 bytes"</a></li>
        <li><a href="ubifs.html#L_study_ubifs">I want to study UBIFS - any recommendations?</a></li>
 </ol>
 
@@ -1110,6 +1111,38 @@ correct LEB size via the <code>-e</code> option.</p>
 
 
 
+<h2><a name="L_ecc_error">
+I see this UBI error: "ubi_io_read: error -74 while reading 126976 bytes from PEB 47:4096, read 126976 bytes"
+</a></h2>
+
+<p>The <code>-74</code> error code is <code>-EBADMSG</code> and means an ECC
+error. In other words, UBI tried to read some data from the flash, but the
+flash driver found that there is an uncorrectable ECC error, and returned
+<code>-EBADMSG</code>.</p>
+
+<p>There may be many reasons for this. It may be because your NAND driver is
+buggy, or you HW is buggy. We recommend you to validate the driver using the
+<a href="../doc/general.xml#L_mtd_tests">MTD tests</a>.</p>
+
+<p>The other possibility is that you failed to flash your UBI/UBIFS image
+properly. Try to erase your flash, then attach it to UBI/UBIFS without writing
+any image, and check if you still have these errors.</p>
+
+<p>If you do not have errors when you mount empty flash, it is probably indeed
+related to how you flash the UBI/UBIFS images. One typical problem is related
+to ECC calculation algorithm - read
+<a name="ubifs.html#L_why_ubiformat">here</a> for more information. Make sure
+that you use <a name="../doc/ubifs.html#L_usptools">ubiformat</a>), or make sure
+your flashing program skips 0xFF properly (see
+<a href="../doc/ubi.html#L_flasher_algo">here</a>).</p>
+
+<p>Another possibility is that your flash does reports that it supports
+<a href="../doc/ubi.html#L_subpage">sub-page</a>, but does not actually support
+them properly. In that case the <code>-74</code> error happens when reading the
+first NAND page.</p>
+
+
+
 <h2><a name="L_study_ubifs">I want to study UBIFS - any recommendations?</a></h2>
 
 <p>Follow <a href="../doc/ubifs.html#L_documentation">these</a> instructions.</p>