On 07/23/2013 12:37 PM, Russell King - ARM Linux wrote:
> On Tue, Jul 23, 2013 at 12:00:30PM +0930, Rusty Russell wrote:
>> Michal Simek <mon...@monstr.eu> writes:
>>> Let me take some code from virtio_rpmsg_bus.c to show that problematic part.
>>>
>>> bufs_va = dma_alloc_coherent(vdev->dev.parent->parent,
>>> RPMSG_TOTAL_BUF_SPACE,
>>> &vrp->bufs_dma, GFP_KERNEL);
>>> vrp->rbufs = bufs_va;
>>> for (i = 0; i < RPMSG_NUM_BUFS / 2; i++) {
>>> struct scatterlist sg;
>>> void *cpu_addr = vrp->rbufs + i * RPMSG_BUF_SIZE;
>>>
>>> sg_init_one(&sg, cpu_addr, RPMSG_BUF_SIZE);
>>> }
>>
>> Hmm. Looking at arch/arm/include/asm/memory.h:
>>
>> #define virt_to_page(kaddr) pfn_to_page(__pa(kaddr) >> PAGE_SHIFT)
>> #define virt_addr_valid(kaddr) ((unsigned long)(kaddr) >= PAGE_OFFSET
>> && (unsigned long)(kaddr) < (unsigned long)high_memory)
>>
>> pfn_to_page in ARM seems to be the asm-generic one, which depends on
>> CONFIG_SPARSEMEM/CONFIG_DISCONTIGMEM/CONFIG_FLATMEM etc.
>>
>> Perhaps virt_addr_valid() is wrong for your config? It's pretty clear
>> that you shouldn't call virt_to_page() on something for which
>> !virt_addr_valid().
>
> The above code fragment is just wrong. You can't make any assumptions about
> the memory returned from dma_alloc_coherent(), because:
>
> - On x86, it is kernel direct mapped memory, where things like
> virt_to_phys(cpuaddr) will work fine with it.
>
> - On any architecture which needs to remap memory to make it coherent
> with the DMA device, the key word there is "remap" - it's not kernel
> direct mapped memory, and virt_to_phys(cpuaddr) on it is illegal.
>
> The only valid operation with scatterlists and such memory returned from
> dma_alloc_coherent() is:
>
> sg_dma_address(sg) = dma_address;
> sg_dma_len(sg) = dma_length;
>
> and not to use the virtual address(es) at all in the scatterlist. And
> it is very very important to realise that you must not mix the streaming
> and coherent DMA APIs - you must never pass coherent memory into the
> dma_map_* functions.Ohad: This code is probably yours and you know the whole history of it. Can you please comment it? Thanks, Michal -- Michal Simek, Ing. (M.Eng), OpenPGP -> KeyID: FE3D1F91 w: www.monstr.eu p: +42-0-721842854 Maintainer of Linux kernel - Microblaze cpu - http://www.monstr.eu/fdt/ Maintainer of Linux kernel - Xilinx Zynq ARM architecture Microblaze U-BOOT custodian and responsible for u-boot arm zynq platform
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