[RFC PATCH v4 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU

[Benjamin Gaignard]

2017-01-12 11:35 GMT+01:00 Benjamin Gaignard <benjamin.gaignard at linaro.org>:
> 2017-01-11 15:34 GMT+01:00 Vladimir Murzin <vladimir.murzin at arm.com>:
>> On 11/01/17 13:17, Benjamin Gaignard wrote:
>>> 2017-01-10 15:18 GMT+01:00 Vladimir Murzin <vladimir.murzin at arm.com>:
>>>> Hi,
>>>>
>>>> It seem that addition of cache support for M-class cpus uncovered
>>>> latent bug in DMA usage. NOMMU memory model has been treated as being
>>>> always consistent; however, for R/M classes of cpu memory can be
>>>> covered by MPU which in turn might configure RAM as Normal
>>>> i.e. bufferable and cacheable. It breaks dma_alloc_coherent() and
>>>> friends, since data can stuck in caches now or be buffered.
>>>>
>>>> This patch set is trying to address the issue by providing region of
>>>> memory suitable for consistent DMA operations. It is supposed that
>>>> such region is marked by MPU as non-cacheable. Robin suggested to
>>>> advertise such memory as reserved shared-dma-pool, rather then using
>>>> homebrew command line option, and extend dma-coherent to provide
>>>> default DMA area in the similar way as it is done for CMA (PATCH
>>>> 2/5). It allows us to offload all bookkeeping on generic coherent DMA
>>>> framework, and it is seems that it might be reused by other
>>>> architectures like c6x and blackfin.
>>>>
>>>> Dedicated DMA region is required for cases other than:
>>>>  - MMU/MPU is off
>>>>  - cpu is v7m w/o cache support
>>>>  - device is coherent
>>>>
>>>> In case one of the above conditions is true dma operations are forced
>>>> to be coherent and wired with dma_noop_ops.
>>>>
>>>> To make life easier NOMMU dma operations are kept in separate
>>>> compilation unit.
>>>>
>>>> Since the issue was reported in the same time as Benjamin sent his
>>>> patch [1] to allow mmap for NOMMU, his case is also addressed in this
>>>> series (PATCH 1/5 and PATCH 3/5).
>>>>
>>>> Thanks!
>>>
>>> I have tested this v4 on my setup (stm32f4, no cache, no MPU) and unfortunately
>>> it doesn't work with my drm/kms driver.
>>
>> I guess the same is for fbmem, but would be better to have confirmation since
>> amba-clcd I use has not been ported to drm/kms (yet), so I can't test.
>>
>>> I haven't any errors but nothing is displayed unlike what I have when
>>> using current dma-mapping
>>> code.
>>> I guess the issue is coming from dma-noop where __get_free_pages() is
>>> used instead of alloc_pages()
>>> in dma-mapping.
>>
>> Unless I've missed something bellow is a call stack for both
>>
>> #1
>> __alloc_simple_buffer
>>         __dma_alloc_buffer
>>                 alloc_pages
>>                 split_page
>>                 __dma_clear_buffer
>>                         memset
>>         page_address
>>
>> #2
>> __get_free_pages
>>         alloc_pages
>>         page_address
>>
>> So the difference is that nommu case in dma-mapping.c memzeros memory, handles
>> DMA_ATTR_NO_KERNEL_MAPPING and does optimisation of memory usage.
>>
>> Is something from above critical for your driver?
>
> I have removed all the diff (split_page,  __dma_clear_buffer, memset)
> from #1 and it is still working.
> DMA_ATTR_NO_KERNEL_MAPPING flag is not set when allocating the buffer.
>
> I have investigated more and found that dma-noop doesn't take care of
> "dma-ranges" property which is set in DT.
> I believed that is the root cause of my problem with your patches.

After testing changing virt_to_phys to virt_to_dma in dma-noop.c fix the issue
modetest and fbdemo are now still functional.

>
> Benjamin
>
>>
>>>
>>> Since my hardware doesn't have cache or MPU (and so use dma-noop) I
>>> haven't reserved specific memory region.
>>> Buffer addresses and vma parameters look correct... What could I have
>>> miss here ?
>>
>> No ideas, sorry...
>>
>> Cheers
>> Vladimir
>>
>>>
>>> Benjamin
>>>
>>>>
>>>> [1] http://www.armlinux.org.uk/developer/patches/viewpatch.php?id=8633/1
>>>>
>>>> Vladimir Murzin (5):
>>>>   dma: Add simple dma_noop_mmap
>>>>   drivers: dma-coherent: Introduce default DMA pool
>>>>   ARM: NOMMU: Introduce dma operations for noMMU
>>>>   ARM: NOMMU: Set ARM_DMA_MEM_BUFFERABLE for M-class cpus
>>>>   ARM: dma-mapping: Remove traces of NOMMU code
>>>>
>>>>  .../bindings/reserved-memory/reserved-memory.txt   |   3 +
>>>>  arch/arm/include/asm/dma-mapping.h                 |   3 +-
>>>>  arch/arm/mm/Kconfig                                |   2 +-
>>>>  arch/arm/mm/Makefile                               |   5 +-
>>>>  arch/arm/mm/dma-mapping-nommu.c                    | 252 +++++++++++++++++++++
>>>>  arch/arm/mm/dma-mapping.c                          |  26 +--
>>>>  drivers/base/dma-coherent.c                        |  59 ++++-
>>>>  lib/dma-noop.c                                     |  21 ++
>>>>  8 files changed, 335 insertions(+), 36 deletions(-)
>>>>  create mode 100644 arch/arm/mm/dma-mapping-nommu.c
>>>>
>>>> --
>>>> 2.0.0
>>>>
>>>
>>>
>>>
>>
>
>
>
> --
> Benjamin Gaignard
>
> Graphic Study Group
>
> Linaro.org │ Open source software for ARM SoCs
>
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-- 
Benjamin Gaignard

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