Re: [PATCH v1 1/1] ARM: Select DMA_DIRECT_REMAP to fix restricted DMA

[Robin Murphy]

On 28/09/2023 1:07 pm, Jim Quinlan wrote:
> On Wed, Sep 27, 2023 at 7:10 PM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> > Hi Jim,
> >
> > thanks for your patch!
> >
> > On Tue, Sep 26, 2023 at 7:52 PM Jim Quinlan <james.quinlan@xxxxxxxxxxxx> wrote:
> >
> > > Without this commit, the use of dma_alloc_coherent() while
> > > using CONFIG_DMA_RESTRICTED_POOL=y breaks devices from working.
> > > For example, the common Wifi 7260 chip (iwlwifi) works fine
> > > on arm64 with restricted memory but not on arm, unless this
> > > commit is applied.
> > >
> > > Signed-off-by: Jim Quinlan <james.quinlan@xxxxxxxxxxxx>
> >
> > (...)
> > > +       select DMA_DIRECT_REMAP
> >
> > Christoph invented that symbol so he can certainly
> > explain what is missing to use this on ARM.
> >
> > This looks weird to me, because:
> > > git grep atomic_pool_init
> > arch/arm/mm/dma-mapping.c:static int __init atomic_pool_init(void)
> > kernel/dma/pool.c:static int __init dma_atomic_pool_init(void)
> >
> > Now you have two atomic DMA pools in the kernel,
> > and a lot more than that is duplicated. I'm amazed that it
> > compiles at all.
> >
> > Clearly if you want to do this, surely the ARM-specific
> > arch/arm/mm/dma-mapping.c and arch/arm/mm/dma-mapping-nommu.c
> > needs to be removed at the same time?
> >
> > However I don't think it's that simple, because Christoph would surely
> > had done this a long time ago if it was that simple.
>
> Hello Linus,
>
> Yes, this is the reason I used "RFC" as the fix looked too easy to be viable :-)
> I debugged it enough to see that the host driver's
> writes to the dma_alloc_coherent() region  were not appearing in
> memory, and that
> led me to DMA_DIRECT_REMAP.

Oh, another thing - the restricted-dma-pool is really only for streaming 
DMA - IIRC there can be cases where the emergency fallback of trying to 
allocate out of the bounce buffer won't work properly. Are you also 
using an additional shared-dma-pool carveout to satisfy the coherent 
allocations, per the DT binding?

Thanks,
Robin.