Re: [PATCH v3 3/3] hw/usb/xhci: Always expect 'dma' link property to be set

[Peter Maydell]

On Fri, 27 Aug 2021 at 10:14, Mark Cave-Ayland
<mark.cave-ayland@ilande.co.uk> wrote:
> Ah so the plan moving forward is to always have an explicit MR passed in for 
> DMA use.
> Sorry if I missed that in earlier versions of the patchset, I'm still getting 
> back up
> to speed on QEMU hacking.
>
> Was there a decision as to what the property name should be? I see "dma_mr" 
> used
> quite a bit, and if there will be more patches to remove the system_memory 
> default
> from other devices then it would be nice to try and use the same name 
> everywhere.

No, I don't think we have a convention; it might be nice to add one.
Currently a quick git grep for DEFINE_PROP_LINK and eyeballing of
the results shows that we have:

 "memory" x 7
 "dram" x 4
 "downstream" x 3
 "dma-memory" x 3
 "dma" x 2
 "source-memory"
 "dram-mr"
 "ddr"
 "ddr-ram"
 "gic"
 "cpc"
 "port[N]"
 "dma_mr"
 "ahb-bus"
 "system-memory"
 "main-bus"

This list includes all our TYPE_MEMORY_REGION link properties; a few of these
are special-purpose, and reasonably have specialized names. 2 out of 3 users
of "downstream" are devices which pass on (or filter out) memory transactions
from the CPU (tz-msc, tz-mpc), and I think that name makes sense there.
(The 3rd is pl080.c, which is a plain old DMA controller, and the naming
there is not so well-suited.)

"memory" is mostly SoC and CPU objects taking a link to whatever they should
have as the CPU's view of memory.

I don't have a strong view on what we ought to try to standardize on,
except that I don't like the "_mr" or "-mr" suffix -- I don't think we
need to try to encode the type of the link property in the property name.

It is probably reasonable to have different naming conventions for:
 * SoC and CPU objects, which take a link to the MR which represents
   the CPU/SoC's view of the outside world
 * Endpoint devices which can be DMA masters and take a link giving
   them their view of what they can DMA to
 * filtering/control devices which take incoming transactions from
   an upstream port, filter some and pass the rest through to a
   downstream port

In pretty much all cases, these link properties are used only internally
to QEMU, so if we decide on a naming convention we can fairly easily
rename existing properties to match.

-- PMM