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

[Sascha Hauer <s.hauer@pengutronix.de>]

On Thu, Aug 15, 2024 at 08:57:18AM +0200, Ahmad Fatoum wrote:
> 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?

Ok, reverted it.

Sascha


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