Messages in this thread | | | Subject | Re: [PATCH V2] net: ethernet: mellanox: correct page conversion | From | Timur Tabi <> | Date | Mon, 18 Apr 2016 10:15:56 -0500 |
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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.
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