Why dma_alloc_coherent don't return direct mapped vaddr?

[Arnd Bergmann]

On Thu, Jul 21, 2022 at 5:28 AM Li Chen <me at linux.beauty> wrote:
>
> Hi Arnd,
>
> dma_alloc_coherent two addr:
> 1. vaddr.
> 2. dma_addr
>
> I noticed vaddr is not simply linear/direct mapped to dma_addr, which means I cannot use virt_to_phys/virt_to_page to get
> paddr/page. Instead, I should use dma_addr as paddr and phys_to_page(dma_addr) to get struct page.
>
> My question is why dma_alloc_coherent not simply return phys_to_virt(dma_addr)? IOW, why vaddr is
> not directly mapped to dma_addr?

dma_alloc_coherent() tries to allocate memory that is the correct type
for the device you
pass. If the device is not itself marked as cache coherent in the DT,
then it has to use
an uncached mapping.

The normal linear map of all memory into the kernel address space is
cacheable, so this
device can't use it, and you instead get a new mapping into kernel
space at a different
virtual address.

Note that you should never need a 'struct page' in this case, as the
device needs a physical
ddress and access from kernel space just needs a pointer. Calling
phys_to_page() on
a dma_addr_t is not portable because a lot of devices have DMA
addresses that are not
the same number as the physical address as seen by the CPU, or there
may be an IOMMU
inbetween.

         Arnd