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. > > 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. Yes, as this are registers they must not be accessed directly. However we can use "__attribute__ ((packed, aligned(4)))" to tell the compiler that the base address of this struct is always aligned to 4 bytes. 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 |