On 01/20/2014 01:13 PM, David Laight wrote: > From: >> On 20/01/14 11:58, Marc Kleine-Budde wrote: >>> On 01/20/2014 12:52 PM, Geert Uytterhoeven wrote: >>>> On Mon, Jan 20, 2014 at 12:47 PM, Marc Kleine-Budde wrote: >>>>> On 01/20/2014 12:43 PM, Geert Uytterhoeven wrote: >>>>>> On Thu, Dec 26, 2013 at 10:37 PM, Sergei Shtylyov >>>>>> wrote: >>>>>>> Changes in version 3: >>>>>> >>>>>>> - added '__packed' to 'struct rcar_can_mbox_regs' and 'struct rcar_can_regs'; >>>>>>> - removed unneeded type cast in the probe() method. >>>>>> >>>>>>> +/* Mailbox registers structure */ >>>>>>> +struct rcar_can_mbox_regs { >>>>>>> + u32 id; /* IDE and RTR bits, SID and EID */ >>>>>>> + u8 stub; /* Not used */ >>>>>>> + u8 dlc; /* Data Length Code - bits [0..3] */ >>>>>>> + u8 data[8]; /* Data Bytes */ >>>>>>> + u8 tsh; /* Time Stamp Higher Byte */ >>>>>>> + u8 tsl; /* Time Stamp Lower Byte */ >>>>>>> +} __packed; >>>>>> >>>>>> Sorry, I missed the earlier discussion, but why the _packed? >>>>>> One u32 and 12 bytes makes a nice multiple of 4. >>>>> >>>>> Better safe than sorry, it's the layout of the registers in hardware, >>>>> don't let the compiler optimize here. > > Why not just add a compile time check against the size of the > structure - that will ensure that no padding is accidentally added. And what do when the check fails? Add an the __packed (again)? :D >>>> Actually __packed makes it less safe, as the compiler now assumes >>>> the u32 id member is unaligned, and thus may turn 32-bit accesses into 4 >>>> byte accesses. >>>> >>>> Fortunately it won't happen in this case as the code uses writel/readl to >>>> acces the id member. > > Which means that it will be aligned (and must be aligned). > So the packed is completely useless and pointless. Then we'll remove the packed. Marc -- Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de |