Hi, >-----Original Message----- >From: intel-gvt-dev [mailto:[email protected]] On >Behalf Of Gerd Hoffmann >Sent: Friday, June 02, 2017 11:24 PM >To: Alex Williamson <[email protected]>; Chen, Xiaoguang ><[email protected]> >Cc: Tian, Kevin <[email protected]>; [email protected]; linux- >[email protected]; [email protected]; [email protected]; Lv, >Zhiyuan <[email protected]>; [email protected]; Wang, Zhi >A <[email protected]> >Subject: Re: [PATCH v6 6/6] drm/i915/gvt: Adding interface so user space can >get >the dma-buf > > Hi, > >> > When i915's dma-buf's release() callback is called it will try to >> > free the gem object associated with the dma-buf if its ref count is >> > 0. But in our case the ref count is 1 so no free callback is called >> > so we can not release allocations there. > >Why the ref count is one? The gem object is created by us while creating the dma-buf(the ref count of the gem object is initialized to 1). Later when user import the dma-buf the ref count of the gem object associate with the dma-buf will increased. When user finished using the dma-buf it will decrease the ref count. But the ref count of the gem object will become 1 when all the user finished using the dma-buf because we create the gem object(the test also showing this result).
Typically user only export a dma-buf(no gem object yet) then when user import the dma-buf then a gem object will be created. But in our case we do not implement the dma-buf from scratch but calling the i915_gem_prime_export() where a gem object is an input parameter. Chenxg >Who holds a reference and why? >Maybe it should be the other way around, i.e. the dmabuf holds a reference on >the vgpu instance backing it, i.e. you can't delete the vgpu while dma-bufs >exist? > >> We cannot simply say that the user isn't allowed to release them in >> that order. > >Yep, not going to fly. Can happen even unintentionally because we can pass >around dmabufs to other processes. Example: qemu passes dmabuf to spice- >client, then qemu crashes. mgmt fd is closed before dmabuf fd then. The >kernel >must be able to handle that. > >cheers, > Gerd >_______________________________________________ >intel-gvt-dev mailing list >[email protected] >https://lists.freedesktop.org/mailman/listinfo/intel-gvt-dev
