On 01/20/2014 01:05 PM, Geert Uytterhoeven wrote: > On Mon, Jan 20, 2014 at 1:02 PM, Ben Dooks wrote: >> 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. >>>> >>>> >>>> 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. >> >> I thought we'd gotten past the stage of ever writing register >> definitions as structures? > > It allows to use e.g. "readl(&base->field)", which is used all over the place. > > And sparse will inform you (read: the person who runs it) if you access > it directly. I'm doing this (however just started using additional -D__CHECK_FOO___), before applying patches to my tree. 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 |