On Fri, Feb 19, 2021 at 04:56:16PM +0100, Christian König wrote: > > > Am 19.02.21 um 16:53 schrieb Alan Stern: > > On Fri, Feb 19, 2021 at 02:45:54PM +0100, Christian König wrote: > > > Well as far as I can see this is a relative clear NAK. > > > > > > When a device can't do DMA and has no DMA mask then why it is requesting an > > > sg-table in the first place? > > This may not be important for your discussion, but I'd like to give an > > answer to the question -- at least, for the case of USB. > > > > A USB device cannot do DMA and has no DMA mask. Nevertheless, if you > > want to send large amounts of bulk data to/from a USB device then using > > an SG table is often a good way to do it. The reason is simple: All > > communication with a USB device has to go through a USB host controller, > > and many (though not all) host controllers _can_ do DMA and _do_ have a > > DMA mask. > > > > The USB mass-storage and uas drivers in particular make heavy use of > > this mechanism. > > Yeah, I was assuming something like that would work. > > But in this case the USB device should give the host controllers device > structure to the dma_buf_attach function so that the sg_table can be filled > in with DMA addresses properly. Indeed. Although in the contexts I'm familiar with, the host controller device is actually passed to routines like dma_pool_create, dma_alloc_coherent, dma_map_sg, or dma_map_single. Alan Stern _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
