Hello,

When we allocate cma memory using dma_alloc_attr using
DMA_ATTR_NO_KERNEL_MAPPING attribute. It will return physical address
without virtual mapping and thats the use case of this attribute. but lets
say some vpu/gpu drivers required virtual mapping of some part of the
allocation. then we dont have anything to remap that allocated memory to
virtual memory. and in 32-bit system it difficult for devices like android
to work all the time with virtual mapping, it degrade the performance.

For Example :

Lets say 4k video allocation required 300MB cma memory but not required
virtual mapping for all the 300MB, its require only 20MB virtually mapped
at some specific use case/point of video, and unmap virtual mapping after
uses, at that time this functions will be useful, it works like ioremap()
for cma_alloc() using dma apis.

/*
         * function call(s) to create virtual map of given physical memory
         * range [base, base+size) of CMA memory.
*/
void *cma_remap(__u32 base, __u32 size)
{
        struct page *page = phys_to_page(base);
        void *virt;

        pr_debug("cma: request to map 0x%08x for size 0x%08x\n",
                        base, size);

        size = PAGE_ALIGN(size);

        pgprot_t prot = get_dma_pgprot(DMA_ATTR, PAGE_KERNEL);

        if (PageHighMem(page)){
                virt = dma_alloc_remap(page, size, GFP_KERNEL, prot,
__builtin_return_address(0));
        }
        else
        {
                dma_remap(page, size, prot);
                virt = page_address(page);
        }

        if (!virt)
                pr_err("\x1b[31m" " cma: failed to map 0x%08x" "\x1b[0m\n",
                                base);
        else
                pr_debug("cma: 0x%08x is virtually mapped to 0x%08x\n",
                                base, (__u32) virt);

        return virt;
}

/*
         * function call(s) to remove virtual map of given virtual memory
         * range [virt, virt+size) of CMA memory.
*/

void cma_unmap(void *virt, __u32 size)
{
        size = PAGE_ALIGN(size);
        unsigned long pfn = virt_to_pfn(virt);
        struct page *page = pfn_to_page(pfn);

                if (PageHighMem(page))
                        dma_free_remap(virt, size);
                else
                        dma_remap(page, size, PAGE_KERNEL);

        pr_debug(" cma: virtual address 0x%08x is unmapped\n",
                        (__u32) virt);
}

This functions should be added in arch/arm/mm/dma-mapping.c file.

Please let me know if i am missing anything.

Regards,
Pankaj