Subject: Re: [net-next PATCH] net: phy: aquantia: drop wrong endianness conversion for addr and CRC

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 22 Nov 2023 17:24:33 +0000

On Wed, Nov 22, 2023 at 06:08:13PM +0100, Christian Marangi wrote:
> On further testing on BE target with kernel test robot, it was notice
> that the endianness conversion for addr and CRC in fw_load_memory was
> wrong and actually not needed. Values in define doesn't get converted
> and are passed as is and hardcoded values are already in what the PHY
> require, that is LE.
> 
> Also drop the cpu_to_be32 for CRC calculation as it's wrong and use
> _swab32 instead, the word is taked from firmware and is always LE, the

                               taken

> mailbox will emit a BE CRC hence the word needs to be always swapped and
> the endianness of the host needs to be ignored.

I'm not convinced. If the firmware is a bytestream (as most "files" are)
then for val = get_unaligned((u32 *)ptr), where ptr is an array of u8:

ptr[0]	ptr[1]	ptr[2]	ptr[3]	val on LE	val on BE
0x01	0x02	0x03	0x04	0x04030201	0x01020304

So, endianness matters here, and I think as Jakub already suggested, you
need to use get_unaligned_le32().

> diff --git a/drivers/net/phy/aquantia/aquantia_firmware.c b/drivers/net/phy/aquantia/aquantia_firmware.c
> index c5f292b1c4c8..bd093633d0cf 100644
> --- a/drivers/net/phy/aquantia/aquantia_firmware.c
> +++ b/drivers/net/phy/aquantia/aquantia_firmware.c
> @@ -93,9 +93,9 @@ static int aqr_fw_load_memory(struct phy_device *phydev, u32 addr,
>  	u16 crc = 0, up_crc;
>  	size_t pos;
>  
> -	/* PHY expect addr in LE */
> -	addr = (__force u32)cpu_to_le32(addr);
> -
> +	/* PHY expect addr in LE. Hardcoded addr in defines are
> +	 * already in this format.
> +	 */
>  	phy_write_mmd(phydev, MDIO_MMD_VEND1,
>  		      VEND1_GLOBAL_MAILBOX_INTERFACE1,
>  		      VEND1_GLOBAL_MAILBOX_INTERFACE1_CRC_RESET);
> @@ -128,7 +128,7 @@ static int aqr_fw_load_memory(struct phy_device *phydev, u32 addr,
>  		 * We convert word to big-endian as PHY is BE and mailbox will
>  		 * return a BE CRC.
>  		 */
> -		word = (__force u32)cpu_to_be32(word);
> +		word = __swab32(word);
>  		crc = crc_ccitt_false(crc, (u8 *)&word, sizeof(word));

Again, I think you need to be careful with the endianness here again.
>From what I understand here, it seems the CRC needs to be generated by
looking at the byte at ptr[3] first, then ptr[2], ptr[1] and finally
ptr[0] ?

If that is the case, the problem is using __swab32() on LE will do the
job for you, but on BE machines, it will be wrong.

I would make this explicit:

		u8 crc_data[4];

		...

		/* CRC is calculated using BE order */
		crc_data[0] = word >> 24;
		crc_data[1] = word >> 16;
		crc_data[2] = word >> 8;
		crc_data[3] = word;

		crc = crc_ccitt_false(crc, crc_data, sizeof(crc_data));

which will be (a) completely unambiguous, and (b) completely
independent of the host endianness.

-- 
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