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

[Christoph Hellwig <hch@lst.de>]

On Fri, Oct 20, 2023 at 10:16:46AM +0200, Marek Szyprowski wrote:
> For historical reasons (performance and limitations of the pre-ARM v7 
> cores), on the 32bit ARM the whole kernel's direct mapping is done using 
> so called 'sections' (1MiB size afair). Those sections are created in 
> the per-process MMU page tables (there are no separate MMU table for the 
> kernel mappings), so altering those mappings requires updating bits in 
> all processes in the system. Practically this means that those mappings 
> has to be static once created during boot time.

That's actually the same on many architetures, and matches the
explanation I heard from Russell before.

> That's why when no CMA 
> is selected, the whole dma_alloc_coherent() allocations are limited to 
> rather small region, which is already remapped as non-cached during boot.

But this does not match my understanding of the code:

 - arch_dma_alloc calls __dma_alloc with is_coherent set to false
 - __dma_alloc then selects cma_allocator if CMA is supported for
   the device / allocation, else remap_allocator if the allocation
   is allowed to block and only if blocking is not allowed pool_allocator
   to allocate from the boot-time pool

This very match matches the dma-direct flow with DMA_DIRECT_REMAP
selected.  The major exception is the direct mapping of the CMA
allocations done by arm32.