Hi Will, On Tue, Jan 22, 2019 at 11:14 AM Will Deacon <will.deacon@xxxxxxx> wrote: > > On Mon, Jan 21, 2019 at 11:35:30AM +0530, Vivek Gautam wrote: > > On Sun, Jan 20, 2019 at 5:31 AM Will Deacon <will.deacon@xxxxxxx> wrote: > > > On Thu, Jan 17, 2019 at 02:57:18PM +0530, Vivek Gautam wrote: > > > > Adding a device tree option for arm smmu to enable non-cacheable > > > > memory for page tables. > > > > We already enable a smmu feature for coherent walk based on > > > > whether the smmu device is dma-coherent or not. Have an option > > > > to enable non-cacheable page table memory to force set it for > > > > particular smmu devices. > > > > > > Hmm, I must be missing something here. What is the difference between this > > > new property, and simply omitting dma-coherent on the SMMU? > > > > So, this is what I understood from the email thread for Last level > > cache support - > > Robin pointed to the fact that we may need to add support for setting > > non-cacheable > > mappings in the TCR. > > Currently, we don't do that for SMMUs that omit dma-coherent. > > We rely on the interconnect to handle the configuration set in TCR, > > and let interconnect > > ignore the cacheability if it can't support. > > I think that's a bug. With that fixed, can you get what you want by omitting > "dma-coherent"? Based on the discussion on the first patch in this series [1], I can update the series. First thing can be - if QUIRK_NO_DMA is set (i.e. the IOMMU _is_ coherent) then we use a cacheable TCR; So, we may need an additional check for this when setting the TCR. For the second case - IOMMUs that are *not* coherent, i.e ones that are omitting 'dma-coherent' property, anyways have to access the page table directly from memory. We take care of the CPU side of this by allocating non-coherent memory, and making sure that we sync the PTEs from map call. Shouldn't we mark TCR for these IOMMUs as non-cacheable for inner and outer cacheability attribute? [1] https://lore.kernel.org/patchwork/patch/1032939/ Regards Vivek > > Will -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
