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

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Tomas,

On 15.08.24 07:53, Tomas Marek wrote:
> 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.

Thanks for the info. See the patch I just Cc'd you on.

@Sascha, can you drop this one patch again?

Thanks,
Ahmad

> 
> Best regards
> 
> Tomas
> 
>>  }
>>  
>>  static unsigned int efi_i2c_req_op_cnt(
>> -- 
>> 2.39.2
>>
> 


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