Re: [PATCH 5/6] i2c: efi: avoid 64-bit division

[Tomas Marek <tomas.marek@elrest.cz>]

Hello Ahmad,

Thanks for maintenance of the Barebox EFI.

On Wed, Aug 14, 2024 at 11:24:23AM +0200, Ahmad Fatoum wrote:
> I2C message length is 16 bit, so doing a 64-bit division is overkill.
> Let's remove the cast and just rely on the usual integer promotion
> to 32-bit.
> 
> Cc: Tomas Marek <tomas.marek@elrest.cz>
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
>  drivers/i2c/busses/i2c-efi.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-efi.c b/drivers/i2c/busses/i2c-efi.c
> index 5f6cc0eed28e..a666a28fab51 100644
> --- a/drivers/i2c/busses/i2c-efi.c
> +++ b/drivers/i2c/busses/i2c-efi.c
> @@ -114,7 +114,7 @@ static unsigned int efi_i2c_msg_op_cnt(const struct efi_i2c_priv *i2c_priv,
>  
>  	max_len = efi_i2c_max_len(i2c_priv, msg);
>  
> -	return ((u64)msg->len + max_len - 1) / max_len;
> +	return (msg->len + max_len - 1) / max_len;

The issue here is that max_len, which represents the MaximumReceiveBytes
or MaximumTransmitBytes in EFI I2C capabilities, is 32-bit. In some EFI
implementations (including the one I use), the EFI I2C MaximumReceiveBytes
and MaximumTransmitBytes are set to the maximum value (0xFFFFFFFF). This
causes msg->len + max_len - 1 to overflow, leading to an incorrect number
of returned operations. To address this, the msg->len is extended to 64 bits.

Best regards

Tomas

>  }
>  
>  static unsigned int efi_i2c_req_op_cnt(
> -- 
> 2.39.2
>