On 2022-03-21 4:07 a.m., Thomas
Hellström wrote:
On 3/21/22 11:30, Tvrtko Ursulin wrote:
On 19/03/2022 19:42, Michael Cheng wrote:
Previous concern with using drm_clflush_sg was that we don't know what the
sg_table is pointing to, thus the usage of wbinvd_on_all_cpus to flush
everything at once to avoid paranoia.
And now we know, or we know it is not a concern?
To make i915 more architecture-neutral and be less paranoid, lets attempt to
"Lets attempt" as we don't know if this will work and/or what can/will break?
use drm_clflush_sg to flush the pages for when the GPU wants to read
from main memory.
Signed-off-by: Michael Cheng <michael.cheng@xxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
index f5062d0c6333..b0a5baaebc43 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c
@@ -8,6 +8,7 @@
#include <linux/highmem.h>
#include <linux/dma-resv.h>
#include <linux/module.h>
+#include <drm/drm_cache.h>
#include <asm/smp.h>
@@ -250,16 +251,10 @@ static int i915_gem_object_get_pages_dmabuf(struct drm_i915_gem_object *obj)
* DG1 is special here since it still snoops transactions even with
* CACHE_NONE. This is not the case with other HAS_SNOOP platforms. We
* might need to revisit this as we add new discrete platforms.
- *
- * XXX: Consider doing a vmap flush or something, where possible.
- * Currently we just do a heavy handed wbinvd_on_all_cpus() here since
- * the underlying sg_table might not even point to struct pages, so we
- * can't just call drm_clflush_sg or similar, like we do elsewhere in
- * the driver.
*/
if (i915_gem_object_can_bypass_llc(obj) ||
(!HAS_LLC(i915) && !IS_DG1(i915)))
- wbinvd_on_all_cpus();
+ drm_clflush_sg(pages);
And as noticed before, drm_clfush_sg still can call wbinvd_on_all_cpus so are you just punting the issue somewhere else? How will it be solved there?
I think in this case, drm_clflush_sg() can't be immediately used, because pages may not contain actual page pointers; might be just the dma address. It needs to be preceded with a dmabuf vmap.
Could you elaborate more with using a dmabuf vmap?
Doing a quick grep on drm_clflush_sg, were you thinking about
something similar to the following?
if (obj->cache_dirty) {
WARN_ON_ONCE(IS_DGFX(i915));
obj->write_domain = 0;
if (i915_gem_object_has_struct_page(obj))
drm_clflush_sg(pages);
obj->cache_dirty = false;
}
Thanks,
Michael Cheng
But otherwise this change, I figure, falls into the "prefer range-aware apis" category; If the CPU supports it, flush the range only, otherwise fall back to wbinvd().
/Thomas
Regards,
Tvrtko
sg_page_sizes = i915_sg_dma_sizes(pages->sgl);
__i915_gem_object_set_pages(obj, pages, sg_page_sizes);
