Re: [PATCH 2/2] MIPS: Loongson64: Add cache_sync to loongson_dma_map_ops

[James Hogan <jhogan@kernel.org>]

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Hi Huacai,

On Thu, Jan 25, 2018 at 03:09:33PM +0800, Huacai Chen wrote:
> Hi, James and Christoph,
> 
> We have modified arch/mips/loongson64/common/dma-swiotlb.c to make
> Loongson support coherent&non-coherent devices co-exist. You can see
> code here:
> https://github.com/linux-loongson/linux-loongson/commit/3f212e36b2432a7c4a843649e4b79c9c0837d1d2
> When device is non-coherent, we will call dma_cache_sync().
> 
> Then, if dma_cache_sync() is only designed for
> DMA_ATTR_NON_CONSISTENT, what can I use?

How did you allocate the memory? The appropriate generic API for the
type of memory should always be used in drivers over arch specific
stuff.

AFAIK (see e.g. Documentation/DMA-API.txt):
- dma_alloc_coherent() shouldn't require syncing, though it may require
  flushing of write combiners
- dma_alloc_attrs() only requires syncing when DMA_ATTR_NON_CONSISTENT
  is used, otherwise is the same as dma_alloc_coherent()
- guaranteed contiguous memory within PA range (e.g. kmalloc()'d memory
  with the appropriate GFP_DMA flags) can be synced using the
  dma_map_*() and dma_unmap_*() functions.

Cheers
James

> 1, __dma_sync_virtual() and __dma_sync() are both static functions, so
> can't be called in other files.
> 2, mips_dma_cache_sync() is not static, but I think it isn't designed
> to be called directly (So it should be static).
> 3, dma_cache_wback(), dma_cache_inv() and dma_cache_wback_inv() does't
> take a "direction" argument, so I should determine the direction first
> (then I will reimplement __dma_cache_sync for myself).
> So, It seems that I can only use dma_cache_sync().
> 
> Huacai
> 
> On Wed, Jan 24, 2018 at 11:29 PM, Christoph Hellwig <hch@lst.de> wrote:
> > On Wed, Jan 24, 2018 at 03:03:05PM +0000, James Hogan wrote:
> >> I see, that makes sense. Thanks Christoph. I'll assume this patch isn't
> >> applicable then unless Huacai adds some explanation.
> >
> > In addition there also is no driver that can be used on loongsoon
> > that actually calls dma_cache_sync either.
> >

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