Hi Guenter, On Sun, 13 Jul 2014 08:44:24 -0700, Guenter Roeck wrote: > On 07/13/2014 08:17 AM, Jean Delvare wrote: > > I2C block transfers can have a size up to 32 bytes. If starting close > > to the end of the address space, there may not be enough room to write > > that many bytes (on I2C block writes) or not enough bytes to be read > > (on I2C block reads.) In that case, we must shorten the transfer so > > that it does not exceed the address space. > > > > Signed-off-by: Jean Delvare <jdelvare@xxxxxxx> > > Cc: Guenter Roeck <linux@xxxxxxxxxxxx> > > Cc: Wolfram Sang <wsa@xxxxxxxxxxxxx> > > --- > > drivers/i2c/i2c-stub.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > --- linux-3.16-rc4.orig/drivers/i2c/i2c-stub.c 2014-07-12 11:56:30.933096483 +0200 > > +++ linux-3.16-rc4/drivers/i2c/i2c-stub.c 2014-07-13 17:01:02.891235856 +0200 > > @@ -220,6 +220,8 @@ static s32 stub_xfer(struct i2c_adapter > > * We ignore banks here, because banked chips don't use I2C > > * block transfers > > */ > > + if (data->block[0] > 256 - command) /* Avoid overrun */ > > + data->block[0] = 256 - command; > > is it safe to overwrite data->block[0], or should it be something > like the following ? > > if (data->block[0] > 256 - command) /* Avoid overrun */ > len = 256 - command; > else > len = data->block[0]; It's not only safe, it is desired. Otherwise the caller doesn't know this is a short read, and it may take the end of the block buffer for actual data. Check the code in i2c-core.c:i2c_smbus_read_i2c_block_data(), you'll see it uses and even returns block[0]. Same for writes, that's the only way to notify the caller of short writes. > Also, wonder what happens in the real world if anyone does that. > Would the write stop at offset 255, or would it wrap and write from 0 ? Depends on the chip, I've seen both implementations. It doesn't really matter what i2c-stub does, as device drivers should never do that. I just did not want to risk data leak or corruption in case it ever happens. -- Jean Delvare SUSE L3 Support -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
