On Wed, Jan 17, 2018 at 11:49 PM, Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx> wrote: > > On Wed, Jan 17, 2018 at 12:39 AM, Daniel Vetter <daniel@xxxxxxxx> wrote: >> >> On Tue, Jan 16, 2018 at 04:35:59PM -0800, Gurchetan Singh wrote: >> > This is required to use buffers allocated by vgem on AMD and ARM >> > devices. >> > We're experiencing a case where eviction of the cache races with >> > userspace >> > writes. To fix this, flush the cache after retrieving a page. >> > >> > Signed-off-by: Gurchetan Singh <gurchetansingh@xxxxxxxxxxxx> >> > --- >> > drivers/gpu/drm/vgem/vgem_drv.c | 1 + >> > 1 file changed, 1 insertion(+) >> > >> > diff --git a/drivers/gpu/drm/vgem/vgem_drv.c >> > b/drivers/gpu/drm/vgem/vgem_drv.c >> > index 35bfdfb746a7..fb263969f02d 100644 >> > --- a/drivers/gpu/drm/vgem/vgem_drv.c >> > +++ b/drivers/gpu/drm/vgem/vgem_drv.c >> > @@ -112,6 +112,7 @@ static int vgem_gem_fault(struct vm_fault *vmf) >> > break; >> > } >> > >> > + drm_flush_pages(obj->base.dev->dev, &page, 1); >> >> Uh ... what exactly are you doing? >> >> Asking because the entire "who's responsible for coherency" story is >> entirely undefined still when doing buffer sharing :-/ What is clear is >> that currently vgem entirely ignores this (there's not >> begin/end_cpu_access callback), mostly because the shared dma-buf support >> in drm_prime.c also entirely ignores this. > > > > This patch isn't trying to address the case of a dma-buf imported into vgem. > It's trying to address the case when a buffer is created by > vgem_gem_dumb_create, mapped by vgem_gem_dumb_map and then accessed by user > space. Since the page retrieved by shmem_read_mapping_page during the page > fault may still be in the cache, we're experiencing incorrect data in > buffer. Here's the test case we're running: > > https://chromium.googlesource.com/chromiumos/platform/drm-tests/+/master/vgem_test.c 404s over here (Internal url?). > It fails on line 210 on AMD and ARM devices (not Intel though). So you _do_ import it on the other device driver as a dma-buf (and export it from vgem)? Because coherency isn't well-defined for dma-buf no matter who the exporter/importer is. >> And doing a one-time only >> flushing in your fault handler is definitely not going to fix this (at >> least not if you do anything else than one-shot uploads). > > > There used to be a be vgem_gem_get_pages function, but that's been removed. > I don't know where else to flush in this situation. dma_buf begin/end cpu access. Even exposed as an ioctl when you're using the dma-buf mmap stuff. Vgem doesn't have that, which means as-is the dumb mmap support of vgem can't really support this if you want to do explicit flushing. What would work is uncached/writecombine/coherent dma memory. But then we're in the middle of the entire "who's responsible. -Daniel > >> >> -Daniel >> >> > } >> > return ret; >> > } >> > -- >> > 2.13.5 >> > >> > _______________________________________________ >> > dri-devel mailing list >> > dri-devel@xxxxxxxxxxxxxxxxxxxxx >> > https://lists.freedesktop.org/mailman/listinfo/dri-devel >> >> -- >> Daniel Vetter >> Software Engineer, Intel Corporation >> http://blog.ffwll.ch > > -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch -- 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
