Re: WARNING: drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c:364 vchiq_prepare_bulk_data

["Arnd Bergmann" <arnd@arndb.de>]

On Mon, Jun 10, 2024, at 00:24, Stefan Wahren wrote:

>
> After that i'm getting the following warning:
>
>    create_pagelist: block 1, DMA address 0x00000000f5f62800 doesn't
> align with PAGE_MASK 0xfffffffffffff000
>
> Then i bisected the issue to this commit:
>
> commit 1c1a429efd4ee8ca244cc2401365c983cda4ed76
> Author: Catalin Marinas <catalin.marinas@arm.com>
> Date:   Mon Jun 12 16:32:01 2023 +0100
>
>      arm64: enable ARCH_WANT_KMALLOC_DMA_BOUNCE for arm64
>
>      With the DMA bouncing of unaligned kmalloc() buffers now in place,
> enable
>      it for arm64 to allow the kmalloc-{8,16,32,48,96} caches.  In addition,
>      always create the swiotlb buffer even when the end of RAM is within the
>      32-bit physical address range (the swiotlb buffer can still be
> disabled on
>      the kernel command line).
>
> So how can the root cause of these warnings be fixed?
> Or is the specific WARN_ON() now obsolete?
>

It appears that the buffer that gets passed to the device
has a start address and length that both come from user
space, and in this case crosses a page boundary, and the
second half is not aligned to ARCH_KMALLOC_MINALIGN.

This means it's not actually a kmalloc buffer that goes
wrong, but userspace passing something that is problematic,
see also the comment above create_pagelist().

If that is a correct interpretation of what is going on,
one way to solve it would be to add the same logic for
vchiq user buffers that we have for kmalloc buffers:
if either the start or the size of the user buffer in
vchiq_irq_queue_bulk_tx_rx() are unaligned, just
allocate a new kernel buffer for the whole thing instead
of pinning the user buffer.

     Arnd