On Fri, Jan 19, 2007 at 11:32:04PM +0100, Alexis Berlemont wrote: > Hi, > > I would like to add some comments on this topic, I am very interested by more > details. > > > Here are a few things to check: > > > > 1. Your memory destination and source addresses are contiguous. You can get > > this using something like: > > > > mem = (void*)__get_free_pages(GFP_KERNEL | GFP_DMA, get_order(size)); > > > > 2. Your source and dest addresses are _bus_ addresses. You can get these > > by: > > > > source = virt_to_bus((u32 *)mem); > > But you must check that your src and dst addresses does not point to > a "vmallocated" buffer (unless you allocated less than PAGE_SIZE and you know > how to convert a virtual kernel address into a physical address, for example > thanks to the function iopa() ). > Consequently, with a 2.6 kernel, the function virt_to_bus() will not work > properly on a virtual address. > > Consequently, if the dst or the src address belongs to a buffer allocated > with > vmalloc() or ioremap(), you will have to use the function iopa() to perform > the translation. > > Besides, I have a question : I do not understand why the 2.6 implementation > of > the function virt_to_bus() is different from the 2.4 version; > ->on 2.4, virt_to_bus() calls iopa() (if CONFIG_PPC4xx is defined),therefore > it works with virtual addresses (vmallocated or ioremapped); > ->on 2.6, virt_to_bus works with logical address (kmallocated or > get_free_pages); > > Do you know why ?
virt_to_bus was never supposed to work on anything else except for directly mapped kernel address. 2.6 version just fixes this for 4xx. And yes, virt_to_bus is deprecated. > > > Ooh - and the 3d thing - cache coherence. I.e.: > > > > dma_cache_inv((u32)mem, (u32)nBytes); > > > > I just add a little precision: > > With PPC405 (no cache coherency integrated), you have to invalidate the cache > of the destination buffer (with dma_cache_inv() )and you have to write back > the cache of the source buffer (with dma_cache_wback() ). But if you do not > want to care about the direction point, you can use dma_cache_inv_wback() > which does both operations. > > Of course, if you work with an "ioremapped" area (either as source or > destination buffer), this functions are useless for this buffer (ioremap() > maps the address range with the flag "PAGE_NO_CACHE"). Guys, please stop reinventing the wheel. There is a documented kernel API which takes care of coherency issues and it should be used for any sort of DMA (be it general purpose DMA controller or external DMA-capable device). PLEASE, read Documentation/DMA-API.txt and use it to obtain DMA-able address for your buffer. -- Eugene _______________________________________________ Linuxppc-embedded mailing list [email protected] https://ozlabs.org/mailman/listinfo/linuxppc-embedded
