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 15:51:31 +0000 |
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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()?
Robin.
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