Re: [PATCH v6 1/2] drm/ttm: Add a flag to allow drivers to skip clear-on-free

[Nirmoy Das]

On 8/20/2024 3:33 PM, Thomas Hellström wrote:
Hi, Nirmoy, Christian

On Fri, 2024-08-16 at 15:51 +0200, Nirmoy Das wrote:
Add TTM_TT_FLAG_CLEARED_ON_FREE, which DRM drivers can set before
releasing backing stores if they want to skip clear-on-free.

Cc: Matthew Auld <matthew.a...@intel.com>
Cc: Thomas Hellström <thomas.hellst...@linux.intel.com>
Suggested-by: Christian König <christian.koe...@amd.com>
Signed-off-by: Nirmoy Das <nirmoy....@intel.com>
Reviewed-by: Christian König <christian.koe...@amd.com>
What happens if two devices share the same global TTM pool
  type and one that does its own clearing. Wouldn't there be a pretty
high chance that the the device that doesn't clear its own pages
allocate non-cleared memory from the pool?


You are right, mixing such devices will poison the global pool. 
Unfortunately, I fully concentrated on single device

use-case. This is problematic mainly because on XE, we are doing clear 
on alloc so from ttm prospective the flag is correct.


A quick option would be to limit this for non-pooled allocations on XE 
or even create a separate pool for XE when have this 
gpu-system-page-clear is enabled.

Thanks for catching this, I will test above options and send patches to 
fix the xe commit.


Regards,

Nirmoy



/Thomas

---
  drivers/gpu/drm/ttm/ttm_pool.c | 18 +++++++++++-------
  include/drm/ttm/ttm_tt.h       |  6 +++++-
  2 files changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_pool.c
b/drivers/gpu/drm/ttm/ttm_pool.c
index 8504dbe19c1a..935ab3cfd046 100644
--- a/drivers/gpu/drm/ttm/ttm_pool.c
+++ b/drivers/gpu/drm/ttm/ttm_pool.c
@@ -222,15 +222,18 @@ static void ttm_pool_unmap(struct ttm_pool
*pool, dma_addr_t dma_addr,
  }
  
  /* Give pages into a specific pool_type */
-static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
*p)
+static void ttm_pool_type_give(struct ttm_pool_type *pt, struct page
*p,
+                              bool cleared)
  {
        unsigned int i, num_pages = 1 << pt->order;
  
-	for (i = 0; i < num_pages; ++i) {
-               if (PageHighMem(p))
-                       clear_highpage(p + i);
-               else
-                       clear_page(page_address(p + i));
+       if (!cleared) {
+               for (i = 0; i < num_pages; ++i) {
+                       if (PageHighMem(p))
+                               clear_highpage(p + i);
+                       else
+                               clear_page(page_address(p + i));
+               }
        }
  
  	spin_lock(&pt->lock);
@@ -394,6 +397,7 @@ static void ttm_pool_free_range(struct ttm_pool
*pool, struct ttm_tt *tt,
                                pgoff_t start_page, pgoff_t
end_page)
  {
        struct page **pages = &tt->pages[start_page];
+       bool cleared = tt->page_flags & TTM_TT_FLAG_CLEARED_ON_FREE;
        unsigned int order;
        pgoff_t i, nr;
  
@@ -407,7 +411,7 @@ static void ttm_pool_free_range(struct ttm_pool
*pool, struct ttm_tt *tt,
  
  		pt = ttm_pool_select_type(pool, caching, order);
                if (pt)
-                       ttm_pool_type_give(pt, *pages);
+                       ttm_pool_type_give(pt, *pages, cleared);
                else
                        ttm_pool_free_page(pool, caching, order,
*pages);
        }
diff --git a/include/drm/ttm/ttm_tt.h b/include/drm/ttm/ttm_tt.h
index 2b9d856ff388..cfaf49de2419 100644
--- a/include/drm/ttm/ttm_tt.h
+++ b/include/drm/ttm/ttm_tt.h
@@ -85,6 +85,9 @@ struct ttm_tt {
         * fault handling abuses the DMA api a bit and dma_map_attrs
can't be
         * used to assure pgprot always matches.
         *
+        * TTM_TT_FLAG_CLEARED_ON_FREE: Set this if a drm driver
handles
+        * clearing backing store
+        *
         * TTM_TT_FLAG_PRIV_POPULATED: TTM internal only. DO NOT
USE. This is
         * set by TTM after ttm_tt_populate() has successfully
returned, and is
         * then unset when TTM calls ttm_tt_unpopulate().
@@ -94,8 +97,9 @@ struct ttm_tt {
  #define TTM_TT_FLAG_EXTERNAL          BIT(2)
  #define TTM_TT_FLAG_EXTERNAL_MAPPABLE BIT(3)
  #define TTM_TT_FLAG_DECRYPTED         BIT(4)
+#define TTM_TT_FLAG_CLEARED_ON_FREE    BIT(5)
  
-#define TTM_TT_FLAG_PRIV_POPULATED	BIT(5)
+#define TTM_TT_FLAG_PRIV_POPULATED     BIT(6)
        uint32_t page_flags;
        /** @num_pages: Number of pages in the page array. */
        uint32_t num_pages;