Re: [PATCH v9 6/7] staging: add simple-fpga-bus

[atull <atull@opensource.altera.com>]

On Thu, 23 Jul 2015, Jason Gunthorpe wrote:

> On Thu, Jul 23, 2015 at 02:55:52PM -0700, Moritz Fischer wrote:
> > Hi Alan,
> > 
> > I saw that your socfpga driver doesn't support the partial reconfig
> > use case (not a big deal).
> > What I currently do for Zynq is if I'm doing a non-partial reconfig is
> > that I disable input
> > level shifters and assert *all* resets while reprogramming in my FPGA
> > manager .write_init() and .write_complete().
> 
> I do this as well, but it is a bit more complex.. FPGA specific code
> has to run around and ensure all DMA is shut off, then we need to make
> sure no CPU issued AXI transactions can happen, then we can tear down
> the FPGA side.
> 
> If the FPGA is torn down while an AXI op is inprogress things go
> sideways, we have to work to prevent that :)
> 
> This happens almost for free, I use DT and the device model to
> disconnect the drivers. The drivers are careful to synchronously fence
> off in-progress DMA. Then drop the DT nodes associated with the
> FPGA, finally the actual FPGA cells can be reset.

Yes, the kernel gives us this almost for free.  That's what I like
about using DT overlays to control FPGA programming.

> 
> > In a partial reconfiguration situation, would I have separate
> > simple-fpga buses for each of the parts that I swap out, each with
> > it's own reset and bitfile attached?
> 
> I'd think of partial reconfiguration as another nested FPGA. The
> resets and so forth could be attached to soft controllers in the
> unswappable part of the FPGA.
> 
> DT nodes have to surround it in some way...
> 
> Jason
> 

Yes, in this way each PR chunk will need its own reset so it
won't wiggle busses and affect the rest of the system during PR.

I noticed that currently simple-fpga-bus.c holds an exclusive
ref of the fpga manager.  This would keep a 2nd pr from being
able to access the same fpga manager, so I'll have to change
it so that simple-fpga-bus.c will  put the ref before exiting
probe.

Alan