On Sat, 18 May 2019 20:11:00 +0200 Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > On Thu, 16 May 2019 08:29:28 +0200 > Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > > > On Wed, 15 May 2019 22:51:58 +0200 > > Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > Don't like the second sentence. How about "It handles neither QDIO > in the common code, nor any device type specific stuff (like channel > programs constructed by the DADS driver)." Sounds good to me (with s/DADS/DASD/ :) > > > A side note: making the subchannel device 'own' the DMA stuff of a > > > ccw device (something that was discussed in the RFC thread) is tricky > > > because the ccw device may outlive the subchannel (all that orphan > > > stuff). > > > > Yes, that's... eww. Not really a problem for virtio-ccw devices (which > > do not support the disconnected state), but can we make DMA and the > > subchannel moving play nice with each other at all? > > > > I don't quite understand the question. This series does not have any > problems with that AFAIU. Can you please clarify? Wait, weren't you saying that there actually is a problem? We seem to have the following situation: - the device per se is represented by the ccw device - the subchannel is the means of communication, and dma is tied to the (I/O ?) subchannel - the machine check handling code may move a ccw device to a different subchannel, or even to a fake subchannel (orphanage handling) The moving won't happen with virtio-ccw devices (as they do not support the disconnected state, which is a prereq for being moved around), but at a glance, this looks like it is worth some more thought. - Are all (I/O) subchannels using e.g. the same dma size? (TBH, that question sounds a bit silly: that should be a property belonging to the ccw device, shouldn't it?) - What dma properties does the fake subchannel have? (Probably none, as its only purpose is to serve as a parent for otherwise parentless disconnected ccw devices, and is therefore not involved in any I/O.) - There needs to be some kind of handling in the machine check code, I guess? We would probably need a different allocation if we end up at a different subchannel? I think we can assume that the dma size is at most 31 bits (since that is what the common I/O layer needs); but can we also assume that it will always be at least 31 bits? My take on this is that we should be sure that we're not digging ourselves a hole that will be hard to get out of again should we want to support non-virtio-ccw in the future, not that the current implementation is necessarily broken.
