[RFC PATCH v3 0/5] ARM: Fix dma_alloc_coherent() and friends for NOMMU

[Benjamin Gaignard]

2017-01-09 14:47 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).
>
> @Benjamin I've tested that mmap is working with amba-clcd and following
> hack on top:
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 76c1ad9..64465c9 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1492,6 +1492,24 @@ __releases(&info->lock)
>         return 0;
>  }
>
> +static unsigned fb_capabilities(struct file *file)
> +{
> +       return NOMMU_MAP_DIRECT | NOMMU_MAP_READ | NOMMU_MAP_WRITE;
> +}
> +
> +static unsigned long get_fb_unmapped_area(struct file *filp,
> +                                  unsigned long addr, unsigned long len,
> +                                  unsigned long pgoff, unsigned long flags)
> +{
> +       struct fb_info * const info = filp->private_data;
> +       unsigned long fb_size = PAGE_ALIGN(info->fix.smem_len);
> +
> +       if (pgoff > fb_size || len > fb_size - pgoff)
> +               return -EINVAL;
> +
> +       return (unsigned long)info->screen_base + pgoff;
> +}
> +
>  static const struct file_operations fb_fops = {
>         .owner =        THIS_MODULE,
>         .read =         fb_read,
> @@ -1503,13 +1521,12 @@ static const struct file_operations fb_fops = {
>         .mmap =         fb_mmap,
>         .open =         fb_open,
>         .release =      fb_release,
> -#ifdef HAVE_ARCH_FB_UNMAPPED_AREA
>         .get_unmapped_area = get_fb_unmapped_area,
> -#endif
>  #ifdef CONFIG_FB_DEFERRED_IO
>         .fsync =        fb_deferred_io_fsync,
>  #endif
>         .llseek =       default_llseek,
> +       .mmap_capabilities = fb_capabilities,
>  };
>
>  struct class *fb_class;
>
> I can see that fb-test-app updates display and addresses returnend with
> dma_alloc_cohernet() and mmap() are the same.

I have push get_fb_unmapped_area() function in fbmem last week:
https://cgit.freedesktop.org/drm/drm-misc/commit/?id=82f42e4cc164ed486c9e2b1b74e65b176830d947
and to the same in drm/kms part:
https://cgit.freedesktop.org/drm/drm-misc/commit/?id=62a0d98a188cc4ebd8ea54b37d274ec20465e464
all this work without need to change mmap_capabilities.

Other noMMU architectures don't need to change mmap_capabilities
function to make fbmem works.
I would like to understand how they do before push for this new function.
It sound like it is an ARM Cortex M specific needs with your new implementation.

If we can't avoid using mmap_capabilities we will have to put it (at
least) in fbmem, drm/kms and v4l2.

Benjamin

> Thanks!
>
> [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

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