On Thu, Jan 04, 2018 at 05:01:16PM +0000, Radu Rendec wrote:
> On Thu, 2018-01-04 at 11:22 -0500, Neil Horman wrote:
> > On Thu, Jan 04, 2018 at 03:42:09PM +0000, Radu Rendec wrote:
> > > I believe the aligned() attribute cannot be used here because the whole
> > > ismt_priv structure is dynamically allocated (the allocation is done at
> > > the beginning of the ismt_probe() function).
> > >
> > > Even with a statically allocated structure, aligning the whole
> > > structure does not guarantee the alignment of the dma_buffer field,
> > > because the field offset within the structure can change (e.g. if other
> > > fields are added, removed, moved around etc.).
> >
> > For reference, you can do it like this:
> >
> > #include <stdlib.h>
> > #include <stdio.h>
> >
> >
> > struct buffer_type { char b[3]; };
> > struct ubuffer_type { char b[3]; } __attribute__((aligned(16)));
> >
> > struct s1 {
> > char foo;
> > struct buffer_type bar;
> > };
> >
> > struct s2 {
> > char foo;
> > struct ubuffer_type bar;
> > };
> >
> > int main(int argc, char **argv)
> > {
> > struct s1 *p1;
> > struct s2 *p2;
> >
> > p1 = malloc(sizeof(struct s1));
> > p2 = malloc(sizeof(struct s2));
> >
> > printf("addr of p1->bar = %p\n", &p1->bar);
> > printf("addr of p2->bar = %p\n", &p2->bar);
> >
> > free(p1);
> > free(p2);
> >
> > return 0;
> > }
>
> Hmm... I just tried this on my workstation (x86, 32bit):
>
> [rrendec@rrendec ~]$ gcc -o align align.c
> [rrendec@rrendec ~]$ ./align
> addr of p1->bar = 0x9358009
> addr of p2->bar = 0x9358028
> [rrendec@rrendec ~]$ ./align
> addr of p1->bar = 0x856b009
> addr of p2->bar = 0x856b028
>
> The most significant 4 digits are always different (probably due to
> address randomization), but the least significant 3 are consistently
> 0x009 and 0x028, neither of which is 16 byte aligned.
>
> But I think this is normal, because malloc() gets just a size parameter
> and has no knowledge of the intended data type of the memory area that
> it allocates. It's probably the same with kmalloc() and friends.
>
Yeah, I was under the impression that malloc provided memory in blocks aligned
to a maximum arch specific alignment, and then gcc could just pad the struct
size to guarantee a more granular alignment, but on doing some more reading,
alignment support only applies to bss and data storage, at least for the gcc
linker. PTR_ALIGN is the way to go here.
> > Though as I look at that, you're probably right, using PTR_ALIGN would probably
> > be prettier. I'd be ok with that approach
>
> That's great. I'll prepare an updated patch with PTR_ALIGN and post it
> shortly.
>
Thanks!
Neil
> Thanks,
> Radu
>
>
