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From:   Gregor Herburger <gregor.herburger@ew.tq-group.com>
To:     york.sun@nxp.com, bp@alien8.de, mchehab@kernel.org,
        tony.luck@intel.com, james.morse@arm.com, rrichter@marvell.com
Cc:     linux-edac@vger.kernel.org, linux-kernel@vger.kernel.org,
        Gregor Herburger <gregor.herburger@ew.tq-group.com>
Subject: [PATCH v2 1/1] edac: fsl_ddr_edac: fix expected data message
Date:   Thu, 27 Aug 2020 09:56:00 +0200
Message-Id: <20200827075600.22335-1-gregor.herburger@ew.tq-group.com>
In-Reply-To: <20200817095302.GD549@zn.tnic>
References: <20200817095302.GD549@zn.tnic>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When a correctable single bit error occurs, the driver calculates the
bad_data_bit respectively the bad_ecc_bit. If there is no error in the
corresponding data, the value becomes -1. With this the expected data
message is calculated.

In the case of an error in the lower 32 bits or no error (-1) the right
side operand of the bit-shift becomes negative which is undefined
behavior.

This can result in wrong and misleading messages like this:
[  311.103794] EDAC FSL_DDR MC0: Faulty Data bit: 36
[  311.108490] EDAC FSL_DDR MC0: Expected Data / ECC:   0xffffffef_ffffffff / 0x80000059
[  311.116135] EDAC FSL_DDR MC0: Captured Data / ECC:   0xffffffff_ffffffef / 0x59

Fix this by only calculating the expected data where the error occurred.

With the fix the dmesg output looks like this:
[  311.103794] EDAC FSL_DDR MC0: Faulty Data bit: 36
[  311.108490] EDAC FSL_DDR MC0: Expected Data / ECC:   0xffffffef_ffffffef / 0x59
[  311.116135] EDAC FSL_DDR MC0: Captured Data / ECC:   0xffffffff_ffffffef / 0x59

Signed-off-by: Gregor Herburger <gregor.herburger@ew.tq-group.com>
---
 drivers/edac/fsl_ddr_edac.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/edac/fsl_ddr_edac.c b/drivers/edac/fsl_ddr_edac.c
index 6d8ea226010d..4b6989cf1947 100644
--- a/drivers/edac/fsl_ddr_edac.c
+++ b/drivers/edac/fsl_ddr_edac.c
@@ -343,9 +343,9 @@ static void fsl_mc_check(struct mem_ctl_info *mci)
 
 		fsl_mc_printk(mci, KERN_ERR,
 			"Expected Data / ECC:\t%#8.8x_%08x / %#2.2x\n",
-			cap_high ^ (1 << (bad_data_bit - 32)),
-			cap_low ^ (1 << bad_data_bit),
-			syndrome ^ (1 << bad_ecc_bit));
+			(bad_data_bit > 31) ? cap_high ^ (1 << (bad_data_bit - 32)) : cap_high,
+			(bad_data_bit <= 31) ? cap_low ^ (1 << (bad_data_bit)) : cap_low,
+			(bad_ecc_bit != -1) ? syndrome ^ (1 << (bad_ecc_bit)) : syndrome);
 	}
 
 	fsl_mc_printk(mci, KERN_ERR,
-- 
2.17.1