Re: seems ttm_bo_handle_move_mem not flush cache, if BO move SYS->VRAM?

[Thomas Hellstrom <thomas@shipmail.org>]

On 12/02/2010 03:49 AM, april wrote:
> thanks Thomas
>
> I want to flush CPU cache, before use DMA  from system memory to VRAM.
>
> Seems I should do this in driver move function itself.
>    

Yes. That's the right way.

> bty,
> when BO changes from cached to write-combined, CPU cache will be
> flushed, and all the pages in this BO will do this.
> but, may be only a small part of page's cache flush is really
> needed(CPU just write to a few pages in BO).
>
> Is there any ways to solve this?
>    

Unfortunately not at the moment. It would require tracking of dirty 
pages in TTM and that is on my to-do list, but that also requries 
changes to the linux kernel VM system. At the moment I'm not completely 
sure how costly a clflush is for a cache line that's not in the CPU cache.

For BOs that are almost only written to, you should try to keep also 
system pages write-combined. In that case they are allocated 
write-combined from the TTM memory pool and the cache changing doesn't 
need to occur.

/Thomas

> or there's no need to do this? (clflush those pages that not in cpu
> cache not cost too much?)
>
>
> Thank you
>
> 2010/12/2 Thomas Hellstrom<thomas@shipmail.org>:
>    
>> On 12/01/2010 04:58 AM, april wrote:
>>      
>>> hi all:
>>>
>>> I have a question:
>>>
>>> If a BO in VRAM (WC) evict to SYS memory(may be cached),  and user
>>> process still can access it event it is in SYS memory (may be cached)
>>> .
>>> when this BO volidate to VRAM,  It seems "ttm_bo_handle_move_mem"  not
>>> flush cache (If evict to SYS memory with cached).
>>>
>>>   But flush cache is needed, If driver use DMA to handle BO
>>> move(SYS->VRAM), should "ttm_bo_handle_move_mem" handle this? or
>>> driver do it in "driver->move_notify" before "driver->move" called?
>>>
>>>
>>>        
>> April, in order to understand your proble, Can you be a bit more specific
>> about.
>>
>> 1) Which cache should be flushed? Cpu cache or GPU cache.
>>
>> 2) Why should it be flushed? For CPU caches, TTM only flushes when
>> transitioning the same page from cached to write-combined. If you use DMA
>> from system memory to VRAM, and need to flush the system pages before
>> performing the DMA operation, that should be done in the driver move
>> function.
>>
>> /Thomas
>>
>>
>>      
>>> Thanks
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>>> dri-devel@lists.freedesktop.org
>>> http://lists.freedesktop.org/mailman/listinfo/dri-devel
>>>
>>>        
>>
>>