Hi Lucas, On Mon, Mar 24, 2014 at 10:19 PM, Lucas Stach <l.stach@xxxxxxxxxxxxxx> wrote: > Hi Alexandre, > > Am Montag, den 24.03.2014, 17:42 +0900 schrieb Alexandre Courbot: >> Hi everyone, > [...] >> >> A few lines of hacks (not included here) are still needed to deal with cached >> mappings triggering external aborts and CPU/GPU memory coherency issues, but I >> hope to understand and address these issues next. > > For the coherency issue part you may want to look at my Nouveau on ARM > series. Most of it never made it upstream, as I lacked the time to work > further on this, but it solves the coherency issue from the kernel. Oh, thanks for pointing this out, it will probably be most useful. Shall I assume the patches at https://www.mail-archive.com/nouveau@xxxxxxxxxxxxxxxxxxxxx/msg13557.html are up-to-date? Would you mind if I include the relevant patches of yours in the next iteration of this series? > > It does so by doing the necessary manual cache flushes/invalidates on > buffer access, so costs some performance. To avoid this you really want > to get writecombined mappings into the kernel<->userspace interface. > Simply mapping the pushbuf as WC/US has brought a 7% performance > increase in OpenArena when I last tested this. This test was done with > only one PCIe lane, so the perf increase may be even better with a more > adequate interconnect. Interestingly if I allow writecombined mappings in the kernel I get faults when attempting the read the mapped area: [ 78.074854] Unhandled fault: external abort on non-linefetch (0x1008) at 0xf003e010 ... [ 78.337862] [<c03491a8>] (nouveau_bo_rd32) from [<c0346374>] (nouveau_fence_update+0x5c/0x80) [ 78.352536] [<c0346374>] (nouveau_fence_update) from [<c03463b0>] (nouveau_fence_done+0x18/0x28) [ 78.367531] [<c03463b0>] (nouveau_fence_done) from [<c02b852c>] (ttm_bo_wait+0x104/0x184) [ 78.381915] [<c02b852c>] (ttm_bo_wait) from [<c034c718>] (nouveau_gem_ioctl_cpu_prep+0x40/0xe8) [ 78.396849] [<c034c718>] (nouveau_gem_ioctl_cpu_prep) from [<c029fd5c>] (drm_ioctl+0x404/0x4b8) [ 78.411790] [<c029fd5c>] (drm_ioctl) from [<c0343960>] (nouveau_drm_ioctl+0x54/0x80) [ 78.425805] [<c0343960>] (nouveau_drm_ioctl) from [<c00ea5ec>] (do_vfs_ioctl+0x3f0/0x5bc) [ 78.440277] [<c00ea5ec>] (do_vfs_ioctl) from [<c00ea7ec>] (SyS_ioctl+0x34/0x5c) [ 78.453918] [<c00ea7ec>] (SyS_ioctl) from [<c000e5a0>] (ret_fast_syscall+0x0/0x30) To avoid these I need to set the VRAM default_caching to TTM_PL_FLAG_UNCACHED. It is not clear to me why this is needed. The BO being accessed through the BAR, they are correctly considered as IO memory and mapped using ttm_bo_ioremap(), so it really seems to be unhappy with the WC mapping itself. Note that if I go ahead and force the use of pgprot_writecombine() in ttm_io_prot() to get writecombined user-space mappings, pure DRM programs that map a buffer and try to read it fail similarly, while Mesa's glReadPixels() seems to be happy. I'm not sure what it does differently here. Cheers, Alex. -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
