From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Vetter Subject: Re: [PATCH v3 1/4] dma-buf: add support for virtio exported objects Date: Thu, 14 May 2020 14:30:07 +0200 Message-ID: <20200514123007.GP206103@phenom.ffwll.local> References: <20200311112004.47138-1-stevensd@chromium.org> <20200311112004.47138-2-stevensd@chromium.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-media-owner@vger.kernel.org To: David Stevens Cc: Daniel Vetter , Tomasz Figa , Gerd Hoffmann , David Airlie , "Michael S . Tsirkin" , Jason Wang , Sumit Semwal , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , Linux Kernel Mailing List , dri-devel , "open list:VIRTIO CORE, NET..." , "open list:DMA BUFFER SHARING FRAMEWORK" , "moderated list:DMA BUFFER SHARING FRAMEWORK" , virtio-dev@lists.oasis-open.org List-Id: virtualization@lists.linuxfoundation.org On Thu, May 14, 2020 at 05:19:40PM +0900, David Stevens wrote: > Sorry for the duplicate reply, didn't notice this until now. > > > Just storing > > the uuid should be doable (assuming this doesn't change during the > > lifetime of the buffer), so no need for a callback. > > Directly storing the uuid doesn't work that well because of > synchronization issues. The uuid needs to be shared between multiple > virtio devices with independent command streams, so to prevent races > between importing and exporting, the exporting driver can't share the > uuid with other drivers until it knows that the device has finished > registering the uuid. That requires a round trip to and then back from > the device. Using a callback allows the latency from that round trip > registration to be hidden. Uh, that means you actually do something and there's locking involved. Makes stuff more complicated, invariant attributes are a lot easier generally. Registering that uuid just always doesn't work, and blocking when you're exporting? -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch