[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.auld at intel.com>
>> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>> Suggested-by: Christian König <christian.koenig at amd.com>
>> Signed-off-by: Nirmoy Das <nirmoy.das at intel.com>
>> Reviewed-by: Christian König <christian.koenig at 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;