Re: [RFC/PATCHv2 v2 2/4] dma-mapping: Add dma_remap() APIs

[Laura Abbott]

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@xxxxxxxxxxxxxx>
> > > >
> > > > 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