Messages in this thread | | | Subject | Re: [RFC/PATCHv2 v2 2/4] dma-mapping: Add dma_remap() APIs | From | Laura Abbott <> | Date | Wed, 27 Apr 2016 11:16:32 -0700 |
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On 04/27/2016 08:25 AM, Catalin Marinas wrote: > On Fri, Apr 22, 2016 at 05:35:16PM -0700, Stephen Boyd wrote: >> Quoting Catalin Marinas (2016-04-21 03:35:12) >>> On Tue, Apr 19, 2016 at 06:04:27PM -0700, Stephen Boyd wrote: >>>> From: Laura Abbott <lauraa@codeaurora.org> >>>> >>>> Some systems are memory constrained but they need to load very >>>> large firmwares. The firmware subsystem allows drivers to request >>>> this firmware be loaded from the filesystem, but this requires >>>> that the entire firmware be loaded into kernel memory first >>>> before it's provided to the driver. This can lead to a situation >>>> where we map the firmware twice, once to load the firmware into >>>> kernel memory and once to copy the firmware into the final >>>> resting place. >>>> >>>> This design creates needless memory pressure and delays loading >>>> because we have to copy from kernel memory to somewhere else. >>>> Let's add a couple DMA APIs that allow us to map DMA buffers into >>>> the CPU's address space in arbitrary sizes. With this API, we can >>>> allocate a DMA buffer with DMA_ATTR_NO_KERNEL_MAPPING and move a >>>> small mapping window across our large DMA buffer to load the >>>> firmware directly into buffer. >>> >>> The first two patches in this series don't make sense to me. I don't >>> understand what the memory pressure is: physical or virtual? Because >>> they don't seem to address the former (the DMA buffer is allocated in >>> full) while the latter doesn't need any addressing at all on arm64, we >>> have plenty of VA space. >>> >>> Why do you even need the coherent DMA API? Can you use the streaming API >>> (map_sg etc.) with a separately allocated buffer? >> >> Hmm I guess I need to add in the patches that show how this is used on >> top of "no-map" DT reserved memory regions. There are some more patches >> that allow us to assigned reserved memory regions with the "no-map" >> attribute to devices and then allocate from those regions using the >> coherent DMA APIs. In the downstream kernel it's called a removed dma >> pool[1]. >> >> So the plan is to wire that all up so that the device can have a >> reserved chunk of memory for the firmware that doesn't exist in the >> kernel's linear memory mappings. Once we have allocated the region, we >> can map it into the kernel's view of memory for a short time so that we >> can load the firmware into it (dma_remap part). Once that's over, we >> want to destroy the mapping so that we 1) don't use any of the kernel's >> virtual memory space (dma_unremap part) to back the buffer and 2) so >> that the secure world can protect the memory from the non-secure world. > > Does the firmware already know about such memory? If yes, I presume the > kernel would have to be told about it and won't try to map it in the > linear mapping. > > At this point, wouldn't a combination of: > > dma_declare_coherent_memory() > dma_alloc_from_coherent() > dma_release_from_coherent() > dma_release_declared_memory() > > work? The removed_alloc() implementation in the link you posted doesn't > seem far from dma_alloc_from_coherent(). The releasing of the declared > memory above would unmap the memory, so there are no VA mappings left. >
The removed alloc was specifically written as a fork of the coherent pool. This was a choice for ease of out of tree maintenance. The better choice here would be to fold those features back into dma-coherent.c if needed.
Thanks, Laura
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