Re: [PATCH v3 06/10] mtd: brcmstb_nand: add SoC-specific support

[Brian Norris <computersforpeace@gmail.com>]

On Thu, May 07, 2015 at 12:01:02PM +0200, Arnd Bergmann wrote:
> On Wednesday 06 May 2015 13:49:10 Brian Norris wrote:
> > On Wed, May 06, 2015 at 09:12:43PM +0200, Arnd Bergmann wrote:
> > > On Wednesday 06 May 2015 10:59:50 Brian Norris wrote:
> > > > +       /*
> > > > +        * Some SoCs integrate this controller (e.g., its interrupt bits) in
> > > > +        * interesting ways
> > > > +        */
> > > > +       if (of_property_read_bool(dn, "brcm,nand-soc")) {
> > > > +               struct device_node *soc_dn;
> > > > +
> > > > +               soc_dn = of_parse_phandle(dn, "brcm,nand-soc", 0);
> > > > +               if (!soc_dn)
> > > > +                       return -ENODEV;
> > > > +
> > > > +               ctrl->soc = devm_brcmnand_probe_soc(dev, soc_dn);
> > > > +               if (!ctrl->soc) {
> > > > +                       dev_err(dev, "could not probe SoC data\n");
> > > > +                       of_node_put(soc_dn);
> > > > +                       return -ENODEV;
> > > > +               }
> > > > +
> > > > +               ret = devm_request_irq(dev, ctrl->irq, brcmnand_irq, 0,
> > > > +                                      DRV_NAME, ctrl);
> > > > +
> > > > +               /* Enable interrupt */
> > > > +               ctrl->soc->ctlrdy_set_enabled(ctrl->soc, true);
> > > > +
> > > > +               of_node_put(soc_dn);
> > > > +       } else {
> > > > +               /* Use standard interrupt infrastructure */
> > > > +               ret = devm_request_irq(dev, ctrl->irq, brcmnand_ctlrdy_irq, 0,
> > > > +                                      DRV_NAME, ctrl);
> > > > +       }
> > > > 
> > > 
> > > It looks to me like this should be handled as a nested irqchip, so the node
> > > you look up gets used as the "interrupt-parent" instead, making the behavior
> > > of this SoC transparent to the nand driver.
> > 
> > You snipped the rest of the patch, which involves more than just IRQ
> > handling. The same registers touch both interrupts and data bus endian
> > configuration, so it can't possibly be done transparently to the NAND
> > driver.
> 
> Anything else in there?

Looks like miscellaneous NAND-related control bits. AXI and APB endian
configuration; several interrupt-enable bits (we only use one for now);
a clock-enable; and some timing test mode bits.

> The bus configuration would just involve writing
> a constant value in some register, right?

I'm not an expert on the Cygnus/iProc chips, but I believe the answer is
no: we *must* reconfigure the bus before and after each data
transaction, because it affects the rest of the core too. Others on the
CC list can probably elaborate.

> Doing that in the irqchip
> is also a bit ugly, but may still be better overall than doing it the
> way you have above.

Well, the Cygnus/iProc case is more complicated as I mention. But I
agree that other cases could be nicer, like bcm63138 (which only has
separate interrupt status/enable registers). Do you expect a new irqchip
driver for every arrangement of registers like this? There are a few
similar chips that have status/enable registers in different orders, and
some that combine them into a single word. Do we really need a new
irqchip driver for each one? I might have done that for bcm63138 and
bcm3384, except that I thought I'd have to fall back to this extra
per-SoC support driver for Cygnus anyway.

> > > We recently merged nested irqdomain support as well, which might help here,
> > > or might not be needed.
> > 
> > I'm not familiar with nested irqdomains. Do they address anything like
> > the above problem?
> 
> The problem that nested irqdomains address is when an interrupt is handled
> by two irqchips, in particular when one irqchip handles a virtual interrupt
> number that was claimed by another irqchip already.

I'll do some reading on that, but it definitely doesn't address the main
problem here.

Brian