On Thu, Apr 29, 2021 at 7:55 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > On Thu, Apr 29, 2021 at 07:26:40AM -0400, Peter Geis wrote: > > Good Morning! I'm trying to implement the MSI workaround in as sane a > > method as possible. > > Do you have a better way to have the kernel only allocate memory in > > the lower regions? > > (Without disabling everything above 4G altogether) > > This chip only supports a maximum of 8G. > > (I know, why they did it while maintaining a 32bit bus is beyond me). > > > Please use dma_alloc_coherent to allocate the memory, the dma_addr_t > return value is the address to be fed to the hardware. Before doing > that set the desired mask. DMA_BIT_MASK(33) would get your 8GB. Thanks! Unfortunately this isn't actually DMA allocation, they were using GFP_DMA32 as a hack to allocate regular kernel memory in the 32 bit range. It's in drivers/irqchip/irq-gic-v3-its.c for reference. The functions are simply kcalloc(), kzalloc(), kzalloc_node(), alloc_pages_node(), and alloc_pages(). I'd prefer not to have to rewrite this driver's entire memory allocation system for one errata. I'm following the code path for dma_alloc_coherent to see if anything sticks out to me though.
