Am 13.07.21 um 22:51 schrieb Daniel Vetter:
intel-gfx-ci realized that something is not quite coherent anymore on
some platforms for our i915+vgem tests, when I tried to switch vgem
over to shmem helpers.
After lots of head-scratching I realized that I've removed calls to
drm_clflush. And we need those. To make this a bit cleaner use the
same page allocation tooling as ttm, which does internally clflush
(and more, as neeeded on any platform instead of just the intel x86
cpus i915 can be combined with).
Unfortunately this doesn't exist on arm, or as a generic feature. For
that I think only the dma-api can get at wc memory reliably, so maybe
we'd need some kind of GFP_WC flag to do this properly.
The problem is that this stuff is extremely architecture specific. So
GFP_WC and GFP_UNCACHED are really what we should aim for in the long term.
And as far as I know we have at least the following possibilities how it
is implemented:
* A fixed amount of registers which tells the CPU the caching behavior
for a memory region, e.g. MTRR.
* Some bits of the memory pointers used, e.g. you see the same memory at
different locations with different caching attributes.
* Some bits in the CPUs page table.
* Some bits in a separate page table.
On top of that there is the PCIe specification which defines non-cache
snooping access as an extension.
Mixing that with the CPU caching behavior gets you some really nice ways
to break a driver. In general x86 seems to be rather graceful, but arm
and PowerPC are easily pissed if you mess that up.
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Cc: "Thomas Hellström" <thomas.hellstrom@xxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Maxime Ripard <mripard@xxxxxxxxxx>
Cc: Thomas Zimmermann <tzimmermann@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Acked-by: Christian könig <christian.koenig@xxxxxxx>
Regards,
Christian.
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 296ab1b7c07f..657d2490aaa5 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -10,6 +10,10 @@
#include <linux/slab.h>
#include <linux/vmalloc.h>
+#ifdef CONFIG_X86
+#include <asm/set_memory.h>
+#endif
+
#include <drm/drm.h>
#include <drm/drm_device.h>
#include <drm/drm_drv.h>
@@ -162,6 +166,11 @@ static int drm_gem_shmem_get_pages_locked(struct drm_gem_shmem_object *shmem)
return PTR_ERR(pages);
}
+#ifdef CONFIG_X86
+ if (shmem->map_wc)
+ set_pages_array_wc(pages, obj->size >> PAGE_SHIFT);
+#endif
+
shmem->pages = pages;
return 0;
@@ -203,6 +212,11 @@ static void drm_gem_shmem_put_pages_locked(struct drm_gem_shmem_object *shmem)
if (--shmem->pages_use_count > 0)
return;
+#ifdef CONFIG_X86
+ if (shmem->map_wc)
+ set_pages_array_wb(shmem->pages, obj->size >> PAGE_SHIFT);
+#endif
+
drm_gem_put_pages(obj, shmem->pages,
shmem->pages_mark_dirty_on_put,
shmem->pages_mark_accessed_on_put);
