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SubjectRe: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL_MAPPING
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On 12/07/2019 19:30, Pankaj Suryawanshi wrote:
> 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.

Hmm, is there any significant reason that this case couldn't be handled
with just get_vm_area() plus dma_mmap_attrs(). I know it's only
*intended* for userspace mappings, but since the basic machinery is there...

Robin.

> /*
> * 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
>

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