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

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Tue, 2024-08-20 at 17:30 +0200, Christian König wrote:
> Am 20.08.24 um 15:33 schrieb Thomas Hellström:
> > 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@intel.com>
> > > Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > > Suggested-by: Christian König <christian.koenig@amd.com>
> > > Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
> > > Reviewed-by: Christian König <christian.koenig@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?
> 
> That's completely unproblematic. The flag indicates that the released
> pages are already cleared, if that isn't the case then the flag 
> shouldn't be set on the TT object.

Yeah, this patch is OK, but the way the follow-up xe patch uses it is
problematic since, AFAICT, xe dma clears on alloc, meaning the pool
pages are not cleared after use.

/Thomas

> 
> If one device clear it's pages and another device doesn't clear it's 
> pages then we would just clear the pages of the device which doesn't
> do 
> it with a hardware DMA.
> 
> Regards,
> Christian.
> 
> > 
> > /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;
>