Hi,
Am 10.06.24 um 12:25 schrieb Robin Murphy:
On 2024-06-10 10:24 am, Phil Elwell wrote:
On Mon, 10 Jun 2024 at 10:20, Arnd Bergmann <arnd@xxxxxxxx> wrote:
On Mon, Jun 10, 2024, at 11:15, Laurent Pinchart wrote:
On Mon, Jun 10, 2024 at 11:00:12AM +0200, Arnd Bergmann wrote:
On Mon, Jun 10, 2024, at 10:26, Phil Elwell wrote:
On Mon, 10 Jun 2024 at 07:00, Arnd Bergmann <arnd@xxxxxxxx> wrote:
Why is swiotlb involved at all? The DMA controller on BCM2837 can
access all RAM that is visible to the ARM cores.
When a device is not cache-coherent and the buffer is not
cache aligned, we now use swiotlb to avoid clobbering data
in the same cache line during DMA synchronization.
We used to rely on kmalloc() returning buffers that are
cacheline aligned, but that was very expensive.
Could we reject buffers provided by userspace that are not
cache-aligned ?
My guess is that this will likely break existing applications,
in which case we cannot.
It's probably a good idea to take a look at what buffers
are actually passed by userspace today. That would also
help decide how we allocate bounce buffers if we have to.
E.g. it's a big difference if the buffers are always
within a few bytes, kilobytes or megabytes.
Arnd
vchiq sends partial cache lines at the start and of reads (as seen
from the ARM host) out of band, so the only misaligned DMA transfers
should be from ARM to VPU. This should not require a bounce buffer.
Hmm, indeed the dma_kmalloc_safe() takes into account that unaligned
DMA_TO_DEVICE does not need bouncing, so it would suggest that
something's off in what vchiq is asking for.
I'm available to debug this further, but i need more guidance here.
At least i extend the output for the error case:
- WARN_ON(len == 0);
- WARN_ON(i && (i != (dma_buffers - 1)) && (len &
~PAGE_MASK));
- WARN_ON(i && (addr & ~PAGE_MASK));
+ if (len == 0)
+ pr_warn_once("%s: sg_dma_len() == 0\n", __func__);
+ else if (i && (i != (dma_buffers - 1)) && (len &
~PAGE_MASK))
+ pr_warn_once("%s: following block not page
aligned\n", __func__);
+ else if (i && (addr & ~PAGE_MASK)) {
+ pr_warn_once("%s: block %u, DMA address %pad
doesn't align with PAGE_MASK 0x%lx\n", __func__, i, &addr, PAGE_MASK);
+ pr_warn_once("sg_dma_is_swiotlb: %d, dma_flags:
%x\n", sg_dma_is_swiotlb(sg), sg->dma_flags);
+ }
Example result:
[ 84.180527] create_pagelist: block 1, DMA address 0x00000000f5f74800
doesn't align with PAGE_MASK 0xfffffffffffff000
[ 84.180553] sg_dma_is_swiotlb: 0, dma_flags: 0
Is this helpful?
Thanks,
Robin.
