On Mon, 20 May 2019 12:21:43 +0200 Cornelia Huck <cohuck@xxxxxxxxxx> wrote: > 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/ :) > Of course! > > > > 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? > No, what I tried to say is: if we tried to make all the dma mem belong to the subchannel device, we would have a problem. It appeared as a tempting opportunity for consolidation, but I decided to not do it. > 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 It is not. When for example a virtio-ccw device talks to the device using a channel program, the dma mem hosting the channel program belongs to the ccw device and not to the subchannel. In fact everything but the stuff in io_priv->dma_area belongs to the ccw device. > - the machine check handling code may move a ccw device to a different > subchannel, or even to a fake subchannel (orphanage handling) > Right! > 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? > Basically nothing changes with mem ownership, except that some bits are dma memory now. Should I provide a more detailed answer to the questions above? > 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? > You mean dma_mas by dma size? > 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. > I agree! Regards, Hali
