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