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.
> 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,
Radu
