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. -- Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation collaborative project.
