On 4/18/2016 11:15 AM, Timur Tabi wrote: > Sinan Kaya wrote: >> >> VMAP allows you to make several pages look contiguous to the CPU. >> It can only be used against logical addresses returned from kmalloc >> or alloc_page. >> >> You cannot take several virtually mapped addresses returned by >> dma_alloc_coherent >> and try to make them virtually contiguous again. >> >> The code happens to work on other architectures by pure luck. AFAIK, >> dma_alloc_coherent >> returns logical addresses on Intel systems until it runs out of DMA memory. >> After >> that intel arch will also start returning virtually mapped addresses and >> this code >> will also fail. ARM64 on the other hand always returns a virtually mapped >> address. >> >> The goal of this code is to allocate a bunch of page sized memory and make >> it look >> contiguous. It is just using the wrong API. The correct API is either >> kmalloc or >> alloc_page map it with dma_map_page not dma_alloc_coherent. >> >> The proper usage of dma_map_page requires code to call dma_sync API in >> correct >> places to be compatible with noncoherent systems. This code is already >> assuming >> coherency. It would be a nice to have dma_sync APIs in right places. There >> is no >> harm in calling dma_sync API for coherent systems as they are no-ops in DMA >> mapping >> layer whereas it is a cache flush for non-coherent systems. > > The text would be a great addition to the patch description. >
I can do that on the next version. -- Sinan Kaya Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
