Re: [PATCH v2 1/1] edac: fsl_ddr_edac: fix expected data message

Borislav Petkov
2020-09-03T11:10:39Z

On Thu, Aug 27, 2020 at 09:56:00AM +0200, Gregor Herburger wrote:
> 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,

But if bad_data_bit is -1, this check above will hit and you'd still
shift by -1, IINM.

How about you fix it properly, clean it up and make it more readable in
the process (pasting the code directly instead of a diff because a diff
is less readable):

if ((err_detect & DDR_EDE_SBE) && (bus_width == 64)) {
sbe_ecc_decode(cap_high, cap_low, syndrome,
&bad_data_bit, &bad_ecc_bit);

if (bad_data_bit != -1) {
if (bad_data_bit > 31)
cap_high ^= 1 << (bad_data_bit - 32);
else
cap_low ^= 1 << bad_data_bit;

fsl_mc_printk(mci, KERN_ERR, "Faulty Data bit: %d\n", bad_data_bit);
fsl_mc_printk(mci, KERN_ERR, "Expected Data: %#8.8x_%08x\n",
cap_high, cap_low);
}

if (bad_ecc_bit != -1) {
fsl_mc_printk(mci, KERN_ERR, "Faulty ECC bit: %d\n", bad_ecc_bit);
fsl_mc_printk(mci, KERN_ERR, "Expected ECC: %#2.2x\n",
syndrome ^ (1 << bad_ecc_bit));
}
}

This way you print only when the respective faulty bits have been
properly found and not print anything otherwise.

Hmm?

--
Regards/Gruss,
Boris.