Messages in this thread | | | From | Tomasz Figa <> | Date | Sat, 26 Sep 2020 17:25:27 +0200 | Subject | Re: [PATCH 17/18] dma-iommu: implement ->alloc_noncoherent |
| |
On Sat, Sep 26, 2020 at 4:14 PM Christoph Hellwig <hch@lst.de> wrote: > > On Fri, Sep 25, 2020 at 06:46:22PM +0000, Tomasz Figa wrote: > > > +static void *iommu_dma_alloc_noncoherent(struct device *dev, size_t size, > > > + dma_addr_t *handle, enum dma_data_direction dir, gfp_t gfp) > > > +{ > > > + if (!gfpflags_allow_blocking(gfp)) { > > > + struct page *page; > > > + > > > + page = dma_common_alloc_pages(dev, size, handle, dir, gfp); > > > + if (!page) > > > + return NULL; > > > + return page_address(page); > > > + } > > > + > > > + return iommu_dma_alloc_remap(dev, size, handle, gfp | __GFP_ZERO, > > > + PAGE_KERNEL, 0); > > > > iommu_dma_alloc_remap() makes use of the DMA_ATTR_ALLOC_SINGLE_PAGES attribute > > to optimize the allocations for devices which don't care about how contiguous > > the backing memory is. Do you think we could add an attrs argument to this > > function and pass it there? > > > > As ARM is being moved to the common iommu-dma layer as well, we'll probably > > make use of the argument to support the DMA_ATTR_NO_KERNEL_MAPPING attribute to > > conserve the vmalloc area. > > We could probably at it. However I wonder why this is something the > drivers should care about. Isn't this really something that should > be a kernel-wide policy for a given system?
There are IOMMUs out there which support huge pages and those can benefit *some* hardware depending on what kind of accesses they perform, possibly on a per-buffer basis. At the same time, order > 0 allocations can be expensive, significantly affecting allocation latency, so for devices which don't care about huge pages anyone would prefer simple single-page allocations. Currently the drivers know the best on whether the hardware they drive would care. There are some decision factors listed in the documentation [1].
I can imagine cases where drivers could not be the best to decide about this - for example, the workload could vary depending on the userspace or a product decision regarding the performance vs allocation latency, but we haven't seen such cases in practice yet.
[1] https://www.kernel.org/doc/html/latest/core-api/dma-attributes.html?highlight=dma_attr_alloc_single_pages#dma-attr-alloc-single-pages
Best regards, Tomasz
| |