On Wed, Jun 26, 2013 at 09:49:51AM +0100, Peter Maydell wrote: > On 26 June 2013 00:38, David Gibson <da...@gibson.dropbear.id.au> wrote: > > On Mon, Jun 24, 2013 at 12:02:39PM +0100, Peter Maydell wrote: > >> On 24 June 2013 11:56, Alexander Graf <ag...@suse.de> wrote: > >> > This looks pretty complicated for something actually quite > >> > simple: All you want is to pass in a number of 64bit values, > >> > rather than 32bit ones, right? > >> > >> Nope. If the device tree blob says #address-cells is 1 > >> and #size-cells is 1, then I want to pass in values to > >> go in 32 bit cells. If it says #address-cells is 2 but > >> #size-cells is still 1, then I want to pass in a value > >> for a 64 bit cell then one for a 32 bit cell. If they're > >> both 2 then I want to pass in values for two 64 bit > >> cells. > > > > Hmm.. the property certainly needs to be constructed that way. But I > > think Alex's point is that you could make the arguments all 64-bit, > > and then truncate them in the generated property. > > Er, the arguments *are* all 64 bits and truncated > in the generated property: > + * @...: 0-terminated list of uint32_t number-of-cells, uint64_t value pairs
Duh, sorry, misread.
That's even worse for the point below. uint32_t / uint64_t pairs,
which will sometimes work if you mess that up, until you get the wrong
platform / parameter combination. And the uint32_ts are things that
could naturally be in just a plain old int or long, which might be
64-bit by default on some ABIs, and the uint64_ts could be addresses
in 32-bit space which would naturally be stored in a 32-bit variable,
but *must* be upcast to 64-bit or again this interface will break
subtly on certain platforms.
This is hair-tearing frustration waiting to happen.
> > There's a bigger problem, though, that exists with both versions. In
> > general people expect integer arguments like this not to care too much
> > about the exact integer type, because it will be promoted to the right
> > thing. Except with varargs it won't. So if you ever have a
> > notionally 64-bit address that happens to fit in a 32-bit variable and
> > you pass that it, this function will be broken. And the worst of it
> > is, it'll work most of the time, until you happen to hit the wrong ABI
> > and parameter combination :(.
>
> Do you have a suggested improvement to the API to avoid this?
After some thought, yes, though it will need gcc extensions:
/* Big fat comment saying not to use this function directly */
int __qemu_fdt_pack_ints(void *fdt, int n, uint64_t[])
{
/* Error if n is not even */
/* take the u64s from the array in pairs, packing as the
previous version */
}
#define qemu_fdt_pack_ints(fdt, ...) \
({ \
uint64_t _tmp[] = { __VA_ARGS__ }; \
__qemu_fdt_pack_ints((fdt), ARRAY_SIZE(_tmp), _tmp); \
})
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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