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

[Robin Murphy <robin.murphy@arm.com>]

On 28/09/2023 1:07 pm, Jim Quinlan wrote:
> On Wed, Sep 27, 2023 at 7:10 PM Linus Walleij <linus.walleij@linaro.org> wrote:
>>
>> Hi Jim,
>>
>> thanks for your patch!
>>
>> On Tue, Sep 26, 2023 at 7:52 PM Jim Quinlan <james.quinlan@broadcom.com> 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@broadcom.com>
>>
>> (...)
>>> +       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.