Re: [PATCH 22/26] dma-buf: add enum dma_resv_usage

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 25.11.21 um 16:59 schrieb Daniel Vetter:
> [SNIP]
>> + *
>> + * For example when asking for WRITE fences then the KERNEL fences are returned
>> + * as well. Similar when asked for READ fences then both WRITE and KERNEL
>> + * fences are returned as well.
>> + */
>> +enum dma_resv_usage {
>> +	/**
>> +	 * @DMA_RESV_USAGE_KERNEL: For in kernel memory management only.
>> +	 *
>> +	 * This should only be used for things like copying or clearing memory
>> +	 * with a DMA hardware engine for the purpose of kernel memory
>> +	 * management.
>> +	 *
>> +         * Drivers *always* need to wait for those fences before accessing the
>> +	 * resource protected by the dma_resv object. The only exception for
>> +	 * that is when the resource is known to be locked down in place by
>> +	 * pinning it previously.
> Should dma_buf_pin also do the wait for kernel fences? I think that would
> also ba e bit clearer semantics in the dma-buf patch which does these
> waits for us.
>
> Or should dma_buf_pin be pipelined and it's up to callers to wait? For kms
> that's definitely the semantics we want, but it's a bit playing with fire
> situation, so maybe dma-buf should get the more idiot proof semantics?

Yeah, good question. I've already added a wait after mapping an 
attachment for static importers.

But for dynamic importers I'm not sure what we want to do. On the one 
hand waiting for moves to finish is certainly the more defensive 
approach on the other hand when you have a dynamic importer you 
absolutely need to handle those dependencies correctly anyway.

Regards,
Christian.