bus_dmamem_alloc doesn't follow alignment specified

[Rajesh Kumar]

Hi,

I am writing a PCI-E driver, where I am trying to allocate DMA buffers. But
seems, like the alignment requirements are not satisfied by
bus_dmamem_alloc function. I am using stable/11 branch.

*Code :*
dma_setup(size, align, alimit) {

printf ("size 0x%x align 0x%x alimit 0x%x\n", size, align, alimit);

if (bus_dma_tag_create(parent, align, 0,
alimit, BUS_SPACE_MAXADDR,
NULL, NULL, size, 1, size,
0, NULL, NULL, &dma_tag)) {
return (ENOMEM);
}

if (bus_dmamem_alloc(dma_tag, &virt_addr,
BUS_DMA_WAITOK | BUS_DMA_ZERO, &dma_map)) {
bus_dma_tag_destroy(dma_tag);
return (ENOMEM);
}

if (bus_dmamap_load(dma_tag, dma_map, virt_addr,
size, dma_cbfn, &arg, BUS_DMA_NOWAIT)) {
bus_dmamem_free(dma_tag, virt_addr, dma_map);
bus_dma_tag_destroy(mw->dma_tag);
return (ENOMEM);
}

printf("dma_addr 0x%lx virt_addr 0x%lx\n", (uint64_t)arg->addr,
(uint64_t)virt_addr);

}

dma_cbfn(void *tmp, bus_dma_segment_t *segs, int nsegs, int error)
{
struct dma_args *arg = (struct dma_args *)tmp;
arg->addr = segs[0].ds_addr;
}

*Logs: *
size 0x100000 align 0x100000 alimit 0xffffffff
dma_addr *0x78e00000* virt_addr *0xfffffe1c3665f000*

As seen above, dma_addr is as per alignment requirement, but virtual
address from bus_dmamem_alloc doesn't seem to satisfy the alignment
requirement).

My understanding is both the DMA addr and the virtual address should be as
per the alignment specified. Can anyone please clarify?

Thanks,
Rajesh.

[Konstantin Belousov]

Post by Rajesh Kumar
Hi,
I am writing a PCI-E driver, where I am trying to allocate DMA buffers. But
seems, like the alignment requirements are not satisfied by
bus_dmamem_alloc function. I am using stable/11 branch.
*Code :*
dma_setup(size, align, alimit) {
printf ("size 0x%x align 0x%x alimit 0x%x\n", size, align, alimit);
if (bus_dma_tag_create(parent, align, 0,
alimit, BUS_SPACE_MAXADDR,
NULL, NULL, size, 1, size,
0, NULL, NULL, &dma_tag)) {
return (ENOMEM);
}
if (bus_dmamem_alloc(dma_tag, &virt_addr,
BUS_DMA_WAITOK | BUS_DMA_ZERO, &dma_map)) {
bus_dma_tag_destroy(dma_tag);
return (ENOMEM);
}
if (bus_dmamap_load(dma_tag, dma_map, virt_addr,
size, dma_cbfn, &arg, BUS_DMA_NOWAIT)) {
bus_dmamem_free(dma_tag, virt_addr, dma_map);
bus_dma_tag_destroy(mw->dma_tag);
return (ENOMEM);
}
printf("dma_addr 0x%lx virt_addr 0x%lx\n", (uint64_t)arg->addr,
(uint64_t)virt_addr);
}
dma_cbfn(void *tmp, bus_dma_segment_t *segs, int nsegs, int error)
{
struct dma_args *arg = (struct dma_args *)tmp;
arg->addr = segs[0].ds_addr;
}
*Logs: *
size 0x100000 align 0x100000 alimit 0xffffffff
dma_addr *0x78e00000* virt_addr *0xfffffe1c3665f000*
As seen above, dma_addr is as per alignment requirement, but virtual
address from bus_dmamem_alloc doesn't seem to satisfy the alignment
requirement).
My understanding is both the DMA addr and the virtual address should be as
per the alignment specified. Can anyone please clarify?

Only bus address is aligned.

For single-segment i.e. contiguous allocations, amd64 could benefit from
using dmap VA instead of kmem_alloc. This would be only an optimization,
not done right now.