[PATCH 2/8] ARM: dma-mapping: implement dma_map_single on top of dma_map_page

[Marek Szyprowski]

Hello,

On Monday, June 27, 2011 4:54 PM Arnd Bergmann wrote:

> On Monday 27 June 2011, Marek Szyprowski wrote:
> > On Friday, June 24, 2011 5:24 PM Arnd Bergmann wrote:
> >
> > > On Monday 20 June 2011, Marek Szyprowski wrote:
> > > > > This also breaks dmabounce when used with a highmem-enabled system
> -
> > > > > dmabounce refuses the dma_map_page() API but allows the
> > > dma_map_single()
> > > > > API.
> > > >
> > > > I really not sure how this change will break dma bounce code.
> > > >
> > > > Does it mean that it is allowed to call dma_map_single() on kmapped
> > > > HIGH_MEM page?
> > >
> > > dma_map_single on a kmapped page already doesn't work, the argument
> needs
> > > to be inside of the linear mapping in order for virt_to_page to work.
> >
> > Then I got really confused.
> >
> > Documentation/DMA-mapping.txt says that dma_map_single() can be used only
> > with kernel linear mapping, while dma_map_page() can be also called on
> > HIGHMEM pages.
> 
> Right, this is true in general.

Ok, so I see no reasons for not implementing dma_map_single() on top of 
dma_map_page() like it has been done in asm-generic/dma-mapping-common.h
 
> > Now, lets go to arch/arm/common/dmabounce.c code:
> >
> > dma_addr_t __dma_map_page(struct device *dev, struct page *page,
> >                 unsigned long offset, size_t size, enum
> dma_data_direction dir)
> > {
> >         dev_dbg(dev, "%s(page=%p,off=%#lx,size=%zx,dir=%x)\n",
> >                 __func__, page, offset, size, dir);
> >
> >         BUG_ON(!valid_dma_direction(dir));
> >
> >         if (PageHighMem(page)) {
> >                 dev_err(dev, "DMA buffer bouncing of HIGHMEM pages "
> >                              "is not supported\n");
> >                 return ~0;
> >         }
> >
> >         return map_single(dev, page_address(page) + offset, size, dir);
> > }
> > EXPORT_SYMBOL(__dma_map_page);
> >
> > Am I right that there is something mixed here? I really don't get why
> there is
> > high mem check in dma_map_page implementation. dma_map_single doesn't
> perform
> > such check and works with kmapped highmem pages...
> >
> > Russell also pointed that my patch broke dma bounch with high mem enabled.
> 
> The version of __dma_map_page that you cited is the one used with dmabounce
> enabled, when CONFIG_DMABOUNCE is disabled, the following version is used:
> 
> static inline dma_addr_t __dma_map_page(struct device *dev, struct page
> *page,
>              unsigned long offset, size_t size, enum dma_data_direction
> dir)
> {
>         __dma_page_cpu_to_dev(page, offset, size, dir);
>         return pfn_to_dma(dev, page_to_pfn(page)) + offset;
> }
> 
> This does not have the check, because the kernel does not need to touch
> the kernel mapping in that case.
> 
> If you pass a kmapped page into dma_map_single, it should also not
> work because of the BUG_ON in ___dma_single_cpu_to_dev -- it warns
> you that you would end up flushing the cache for the wrong page (if any).

Yes, I know that the flow is different when dma bounce is not used. Non-dma
bounce version will still work correctly after my patch. 

However I still don't get how my patch broke dma bounce code with HIGHMEM,
what has been pointed by Russell...

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center