On Mon, Aug 9, 2021 at 7:56 AM Will Deacon will@kernel.org wrote:
On Mon, Aug 02, 2021 at 06:36:04PM -0700, Rob Clark wrote:
On Mon, Aug 2, 2021 at 8:14 AM Will Deacon will@kernel.org wrote:
On Mon, Aug 02, 2021 at 08:08:07AM -0700, Rob Clark wrote:
On Mon, Aug 2, 2021 at 3:55 AM Will Deacon will@kernel.org wrote:
On Thu, Jul 29, 2021 at 10:08:22AM +0530, Sai Prakash Ranjan wrote:
On 2021-07-28 19:30, Georgi Djakov wrote: > On Mon, Jan 11, 2021 at 07:45:02PM +0530, Sai Prakash Ranjan wrote: > > commit ecd7274fb4cd ("iommu: Remove unused IOMMU_SYS_CACHE_ONLY flag") > > removed unused IOMMU_SYS_CACHE_ONLY prot flag and along with it went > > the memory type setting required for the non-coherent masters to use > > system cache. Now that system cache support for GPU is added, we will > > need to set the right PTE attribute for GPU buffers to be sys cached. > > Without this, the system cache lines are not allocated for GPU. > > > > So the patches in this series introduces a new prot flag IOMMU_LLC, > > renames IO_PGTABLE_QUIRK_ARM_OUTER_WBWA to IO_PGTABLE_QUIRK_PTW_LLC > > and makes GPU the user of this protection flag. > > Thank you for the patchset! Are you planning to refresh it, as it does > not apply anymore? >
I was waiting on Will's reply [1]. If there are no changes needed, then I can repost the patch.
I still think you need to handle the mismatched alias, no? You're adding a new memory type to the SMMU which doesn't exist on the CPU side. That can't be right.
Just curious, and maybe this is a dumb question, but what is your concern about mismatched aliases? I mean the cache hierarchy on the GPU device side (anything beyond the LLC) is pretty different and doesn't really care about the smmu pgtable attributes..
If the CPU accesses a shared buffer with different attributes to those which the device is using then you fall into the "mismatched memory attributes" part of the Arm architecture. It's reasonably unforgiving (you should go and read it) and in some cases can apply to speculative accesses as well, but the end result is typically loss of coherency.
Ok, I might have a few other sections to read first to decipher the terminology..
But my understanding of LLC is that it looks just like system memory to the CPU and GPU (I think that would make it "the point of coherence" between the GPU and CPU?) If that is true, shouldn't it be invisible from the point of view of different CPU mapping options?
You could certainly build a system where mismatched attributes don't cause loss of coherence, but as it's not guaranteed by the architecture and the changes proposed here affect APIs which are exposed across SoCs, then I don't think it helps much.
Hmm, the description of the new mapping flag is that it applies only to transparent outer level cache:
+/* + * Non-coherent masters can use this page protection flag to set cacheable + * memory attributes for only a transparent outer level of cache, also known as + * the last-level or system cache. + */ +#define IOMMU_LLC (1 << 6)
But I suppose we could call it instead IOMMU_QCOM_LLC or something like that to make it more clear that it is not necessarily something that would work with a different outer level cache implementation?
BR, -R