Hello,
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.
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.
Best regards
--
Marek Szyprowski
Samsung Poland R&D Center
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