Re: [PATCH] drm/ttm: Set CPU caching mode to cached for BOs being swapped out

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/06/2015 03:33 AM, Michel Dänzer wrote:
> On 05.11.2015 17:47, Thomas Hellstrom wrote:
>> Hi, Michel,
>>
>> On 11/05/2015 09:08 AM, Michel Dänzer wrote:
>>> From: Michel Dänzer <michel.daenzer@amd.com>
>>>
>>> I ran into the BUG_ON in ttm_tt_swapout, presumably the BO being swapped
>>> out was using a write-combined CPU mapping.
>>>
>>> Instead of BUGging out, just set the caching mode to what's needed.
>>>
>>> Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
>>> ---
>>>  drivers/gpu/drm/ttm/ttm_tt.c | 3 ++-
>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/ttm/ttm_tt.c b/drivers/gpu/drm/ttm/ttm_tt.c
>>> index 4e19d0f..c2794eb 100644
>>> --- a/drivers/gpu/drm/ttm/ttm_tt.c
>>> +++ b/drivers/gpu/drm/ttm/ttm_tt.c
>>> @@ -334,7 +334,8 @@ int ttm_tt_swapout(struct ttm_tt *ttm, struct file *persistent_swap_storage)
>>>  	int ret = -ENOMEM;
>>>  
>>>  	BUG_ON(ttm->state != tt_unbound && ttm->state != tt_unpopulated);
>>> -	BUG_ON(ttm->caching_state != tt_cached);
>>> +
>>> +	ttm_tt_set_caching(ttm, tt_cached);
>>>  
>>>  	if (!persistent_swap_storage) {
>>>  		swap_storage = shmem_file_setup("ttm swap",
>> This *is* actually a bug somewhere, since before ttm_tt_swapout,
>> ttm_bo_swapout should have moved out the bo to system and set
>> the correct caching.
> Maybe ttm_bo_swapout needs to check ttm->caching_state explicitly?
> AFAICT it only checks the placement flags, but we allow all caching
> modes for GTT.
>

bo->mem.placement should only have two flags set, one for the current
memory type corresponding to bo->mem.mem_type, the other for the current
caching. If there are more than one caching flag set at this point,
that's a bug. The preferred caching mode is chosen from a set of modes
in ttm_bo_select_caching(), and bo->mem is reassigned to incorporate the
current caching mode after a successful move.

I quickly eyed through the TTM default move functions and they seem to
do the correct thing from what I can tell. Is the radeon ttm driver
assigning bo->mem in a move callback perhaps?

/Thomas


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