Re: Proper way to check for restricted DMA addressing from device driver

[Lucas Stach <l.stach@pengutronix.de>]

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.

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?

Regards,
Lucas