linux-kernel - Re: [PATCH] eeprom: at24: fix the magic value for at24mac402

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Date:   Mon, 27 Nov 2017 20:15:17 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>,
        Claudiu Beznea <Claudiu.Beznea@...rochip.com>
Cc:     linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] eeprom: at24: fix the magic value for at24mac402

Am 27.11.2017 um 13:28 schrieb Bartosz Golaszewski:
> There's an ilog2() expansion in AT24_DEVICE_MAGIC() which rounds down
> the actual size of EUI-48 byte array in at24mac402 eeproms to 4 from 6,
> making it impossible to read it all.
> 
> Fix it by creating the magic value manually with the correct size.
> 
> This patch contains a temporary fix that is suitable for stable
> branches. Eventually we'll probably remove the call to ilog2() while
> converting the magic values to actual structs.
> 
> This fix will cause a warning message to be emitted by probe() for
> at24mac402 chips but theys should be initialized correctly anyway.
> 
> Cc: stable@...r.kernel.org
> Fixes: 0b813658c115 ("eeprom: at24: add support for at24mac series")
> Signed-off-by: Bartosz Golaszewski <brgl@...ev.pl>
> ---
>  drivers/misc/eeprom/at24.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
> index e0b4b36ef010..72adf34627f5 100644
> --- a/drivers/misc/eeprom/at24.c
> +++ b/drivers/misc/eeprom/at24.c
> @@ -141,8 +141,17 @@ static const struct i2c_device_id at24_ids[] = {
>  	{ "24c02",	AT24_DEVICE_MAGIC(2048 / 8,	0) },
>  	{ "24cs02",	AT24_DEVICE_MAGIC(16,
>  				AT24_FLAG_SERIAL | AT24_FLAG_READONLY) },
> -	{ "24mac402",	AT24_DEVICE_MAGIC(48 / 8,
> -				AT24_FLAG_MAC | AT24_FLAG_READONLY) },
> +	/*
> +	 * REVISIT: the size of the EUI-48 byte array is 6 in at24mac402, while
> +	 * the call to ilog2() in AT24_DEVICE_MAGIC() rounds it down to 4. We
> +	 * might want to eventually remove it altogether or maybe even convert
> +	 * the magic values to real structs.
> +	 *
> +	 * For now just create the magic value manually with the right size.
> +	 */
> +	{ "24mac402",	((1 << AT24_SIZE_FLAGS |
> +			 (AT24_FLAG_MAC | AT24_FLAG_READONLY))
> +			 << AT24_SIZE_BYTELEN | (48 / 8)) },
>  	{ "24mac602",	AT24_DEVICE_MAGIC(64 / 8,
>  				AT24_FLAG_MAC | AT24_FLAG_READONLY) },
>  	/* spd is a 24c02 in memory DIMMs */
> 
I'm afraid this won't work. byte_len = BIT(least 5 bits of magic value)
Means byte_len for 24mac402 would be BIT(6) = 64 with your patch.

What we could do, although it seems to be quite ugly: round up the ilog2
value. Instead of ilog2(_len) we could use ilog2((_len) * 2 - 1)

Rgds, Heiner