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

[James Hogan <jhogan@kernel.org>]

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On Thu, Jan 25, 2018 at 07:04:37PM +0800, Huacai Chen wrote:
> If call dma_cache_sync() is incorrect,

(it wouldn't work since the cache_sync callback wouldn't be set).

> then which is the correct way?
> 1, call mips_dma_cache_sync()?
> 2, remove static of __dma_sync_virtual() and call it?

Both take a virtual address, which is obviously not directly usable for
functions which get a DMA handle, so __dma_sync() may be better.

Cheers
James

> 
> Huacai
> 
> On Thu, Jan 25, 2018 at 6:54 PM, James Hogan <jhogan@kernel.org> wrote:
> > On Thu, Jan 25, 2018 at 04:44:36PM +0800, Huacai Chen wrote:
> >> On Thu, Jan 25, 2018 at 3:55 PM, James Hogan <jhogan@kernel.org> wrote:
> >> > 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.
> >
> > I also see there is a dma_sync_*_for_{cpu,device} to avoid "mapping" and
> > "unmapping" the same area repeatedly if it is reused.
> >
> >> Yes, kmalloc()'d memory with the appropriate GFP_DMA flags can be
> >> synced using the dma_map_*() and dma_unmap_*() functions. So,
> >> loongson_dma_map_page()/loongson_dma_unmap_page() (which is the
> >> backend of dma_map_*() and dma_unmap_*()) should call dma_cache_sync()
> >> for non-coherent devices, right?
> >
> > Yeh, I suppose thats effectively what it should do (though probably via
> > an arch specific function rather than dma_cache_sync() itself). Also the
> > sync_*_for_{cpu,device} callbacks should probably do it too. See all the
> > calls to __dma_sync() in the MIPS dma-default.c implementation.
> >
> > Cheers
> > James
> >
> >>
> >> Huacai
> >>
> >> >
> >> > 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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