Subject: dma_alloc_coherent versus streaming DMA, neither works satisfactory

From: [email-masked] (Mike Looijmans)
Date: Thu, 7 May 2015 13:18:08 +0200

?On 29-04-15 15:13, Arnd Bergmann wrote:
> On Wednesday 29 April 2015 14:49:26 Mike Looijmans wrote:
>> On 29-04-15 12:41, Arnd Bergmann wrote:
>>> On Wednesday 29 April 2015 12:33:00 Mike Looijmans wrote:
>>>> On 29-04-15 12:07, Arnd Bergmann wrote:
>>>>> On Wednesday 29 April 2015 11:47:37 Mike Looijmans wrote:
>>>>>> On 29-04-15 11:17, Russell King - ARM Linux wrote:
>>>>>>> The buffer should also be mapped into userspace with the same memory
>>>>>>> type and cache attributes as the kernel side mapping.  If using ACP,
>>>>>>> then you probably want "normal memory, cacheable, writeback, read
>>>>>>> allocate" or in the case of SMP, the same but "read/write allocate".
>>>>>
>>>>> If my suspicion is correct, we should either change arm_coherent_dma_ops()
>>>>> to refer to a different mmap() callback that does the right thing for
>>>>> coherent devices, or change arm_dma_mmap() to look at dev->is_coherent.
>>>>
>>>> Following the route, arch/arm/mm/dma-mapping.c uses pgprot_dmacoherent() which
>>>> is defined in arch/arm/include/asm/pgtable.h and that just returns uncached
>>>> memory.
>>>>
>>>> If you can give me some hints as to what the correct flags would be, I can
>>>> patch my kernel and test it.
>>>
>>> Use the flags that Russell listed above.
>>
>> I would if I had a clue how to do that. For one thing, I don't understand all
>> the L_PTE_... flag juggling that happens here.
>>
>> I also tried just using kmalloc() to allocate the buffer, and then
>> dma_map_single that. This is what the DMA documentation told me to do for
>> non-coherent mappings. This works fine for the streaming-dma mode using the
>> HP, but using dma-coherent this not only fails to work, it tends to completely
>> lock the system.
>>
>> Avoiding the streaming mapping and using only the coherent modes prevents the
>> system locking up, it then doesn't do more harm than just corrupting data in
>> the buffers.
>
> If I understand it right, you basically just skip the 'vma->vm_page_prot =
> __get_dma_pgprot(attrs, vma->vm_page_prot);' step and get the right mapping
> here, i.e. the pgprot value we use for all normal memory.

I reverted all my patches and workarounds. Indeed, the kernel needs a 
"coherent" version of the dma_mmap routine, as the current version will map it 
as non-cachable, resulting in a big performance hit (and nullifying the whole 
idea behind it).

I'll test it further on my 'hardware' and cook up a patch that correctly maps 
the coherent pages.

M.

Kind regards,

Mike Looijmans
System Expert

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