Messages in this thread | | | Subject | Re: Proper way to check for restricted DMA addressing from device driver | From | Robin Murphy <> | Date | Wed, 26 Feb 2020 17:00:40 +0000 |
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On 26/02/2020 4:04 pm, Lucas Stach wrote: > On Mi, 2020-02-26 at 15:51 +0000, Robin Murphy wrote: >> On 26/02/2020 3:44 pm, Lucas Stach wrote: >>> Hi all, >>> >>> I'm currently struggling with how to properly check for restricted DMA >>> addressing from a device driver side. The basic issue I'm facing is >>> that I have a embedded GPU, which isn't able to address all system >>> memory due to interconnect being restricted to 32bit addressing. The >>> limits are properly described in the system device-tree and thus >>> SWIOTLB is working. >>> >>> However graphics buffers are large and graphics drivers really like to >>> keep the dma mapping alive for performance reasons, which means I'm >>> running out of SWIOTLB space pretty easily, aside from the obvious >>> performance implications of SWIOTLB. >>> >>> As 3 out of the maximum 4GB system memory are located in the DMA32 zone >>> and thus located in the GPU addressable space, I just want to avoid >>> allocating graphics buffers outside of the DMA32 zone. >>> >>> To add the DMA32 restriction to my drivers allocations, I need a >>> reliable way from the device driver side to check if the GPU is in such >>> a restricted system. What I'm currently doing in my WIP patch is this: >>> >>> /* >>> * If the GPU is part of a system with only 32bit bus addressing >>> * capabilities, request pages for our SHM backend buffers from the >>> * DMA32 zone to avoid performance killing SWIOTLB bounce buffering. >>> */ >>> if (*gpu->dev->dma_mask < BIT_ULL(32) && !device_iommu_mapped(gpu->dev)) >>> priv->shm_gfp_mask |= GFP_DMA32; >>> >>> However I'm not sure if there are edge cases where this check would >>> fool me. Is there any better way to check for DMA addressing >>> restrictions from the device driver side? >> >> dma_addressing_limited()? > > While amdgpu and radeon do seem to use this to trigger DMA32 > allocations, the bool return value doesn't really tell if DMA32 is > helpful or not. All it tells is that the system has memory outside of > the device dma addressing capabilities.
If the limit falls somewhere between 32 bits and "everything" then DMA32 is the best you can do anyway; if it's *less* than 32 bits then you'd be into the realm of "serious problems" already. There is a point at which it's reasonable to assume you don't have to support contrived pathological cases :) > Is the return value of this function correct if the device is behind a > IOMMU? While the device might be on a bus that is address limited a > IOMMU further up the path to memory might allow to address all system > memory, no?
It should be OK - see the comment in dma_get_required_mask(). Obviously it's theoretically possible to integrate a device with 32+ bits of DMA capability to an IOMMU input 32+ bits wide, via an interconnect less than 32 bits wide, but any designer silly enough to actually do that is welcome to have their SoC never supported by Linux.
Robin.
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