From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sinan Kaya Subject: Re: [PATCH V2] net: ethernet: mellanox: correct page conversion Date: Mon, 18 Apr 2016 11:22:05 -0400 Message-ID: <5714FB9D.4000405@codeaurora.org> References: <1460845412-13120-1-git-send-email-okaya@codeaurora.org> <20160418121247.GA25387@infradead.org> <0c6a430c5f0ec64f51d7c594ef9751dd@codeaurora.org> <20160418131058.GA25421@infradead.org> <5714E5D6.7050600@codeaurora.org> <5714FA2C.4030209@codeaurora.org> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, cov-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, Yishai Hadas , netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Timur Tabi , Christoph Hellwig Return-path: In-Reply-To: <5714FA2C.4030209-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: netdev.vger.kernel.org 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 -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html