On Tue, Jun 11, 2024, at 12:47, Stefan Wahren wrote:
> 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:
>>>> 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?
It's interesting that this does not have the SG_DMA_SWIOTLB
flag set, as the theory so far was that an unaligned
user address is what caused this to bounce.
I think the most helpful bit of information that is
currently missing is the 'ubuf' and 'count' arguments
that got passed down from userspace into create_pagelist(),
to see what alignment they have in the failure case.
Arnd
