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.
> 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).
Arnd
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