linux-kernel - Re: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL

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Message-ID: <57f8aa35-d460-9933-a547-fbf578ea42d3@arm.com>
Date:   Tue, 16 Jul 2019 13:02:19 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Pankaj Suryawanshi <pankajssuryawanshi@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        pankaj.suryawanshi@...fochips.com, minchan@...nel.org,
        minchan.kim@...il.com, Christoph Hellwig <hch@....de>
Subject: Re: cma_remap when using dma_alloc_attr :- DMA_ATTR_NO_KERNEL_MAPPING

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
>