On Tue, Feb 02, 2021 at 11:56:27AM +0530, Sai Prakash Ranjan wrote: > On 2021-02-01 23:50, Jordan Crouse wrote: > > On Mon, Feb 01, 2021 at 08:20:44AM -0800, Rob Clark wrote: > > > On Mon, Feb 1, 2021 at 3:16 AM Will Deacon <will@xxxxxxxxxx> wrote: > > > > On Fri, Jan 29, 2021 at 03:12:59PM +0530, Sai Prakash Ranjan wrote: > > > > > On 2021-01-29 14:35, Will Deacon wrote: > > > > > > On Mon, Jan 11, 2021 at 07:45:04PM +0530, Sai Prakash Ranjan wrote: > > > > > > > +#define IOMMU_LLC (1 << 6) > > > > > > > > > > > > On reflection, I'm a bit worried about exposing this because I think it > > > > > > will > > > > > > introduce a mismatched virtual alias with the CPU (we don't even have a > > > > > > MAIR > > > > > > set up for this memory type). Now, we also have that issue for the PTW, > > > > > > but > > > > > > since we always use cache maintenance (i.e. the streaming API) for > > > > > > publishing the page-tables to a non-coheren walker, it works out. > > > > > > However, > > > > > > if somebody expects IOMMU_LLC to be coherent with a DMA API coherent > > > > > > allocation, then they're potentially in for a nasty surprise due to the > > > > > > mismatched outer-cacheability attributes. > > > > > > > > > > > > > > > > Can't we add the syscached memory type similar to what is done on android? > > > > > > > > Maybe. How does the GPU driver map these things on the CPU side? > > > > > > Currently we use writecombine mappings for everything, although there > > > are some cases that we'd like to use cached (but have not merged > > > patches that would give userspace a way to flush/invalidate) > > > > > > > LLC/system cache doesn't have a relationship with the CPU cache. Its > > just a > > little accelerator that sits on the connection from the GPU to DDR and > > caches > > accesses. The hint that Sai is suggesting is used to mark the buffers as > > 'no-write-allocate' to prevent GPU write operations from being cached in > > the LLC > > which a) isn't interesting and b) takes up cache space for read > > operations. > > > > Its easiest to think of the LLC as a bonus accelerator that has no cost > > for > > us to use outside of the unfortunate per buffer hint. > > > > We do have to worry about the CPU cache w.r.t I/O coherency (which is a > > different hint) and in that case we have all of concerns that Will > > identified. > > > > For mismatched outer cacheability attributes which Will mentioned, I was > referring to [1] in android kernel. I've lost track of the conversation here :/ When the GPU has a buffer mapped with IOMMU_LLC, is the buffer also mapped into the CPU and with what attributes? Rob said "writecombine for everything" -- does that mean ioremap_wc() / MEMREMAP_WC? Finally, we need to be careful when we use the word "hint" as "allocation hint" has a specific meaning in the architecture, and if we only mismatch on those then we're actually ok. But I think IOMMU_LLC is more than just a hint, since it actually drives eviction policy (i.e. it enables writeback). Sorry for the pedantry, but I just want to make sure we're all talking about the same things! Cheers, Will
