Subject: Re: [PATCH 04/11] of: address: Preserve the flags portion on 1:1 dma-ranges mapping

From: Andrea della Porta <[email-masked]>
Date: Wed, 21 Aug 2024 10:18:59 +0200

Hi Rob,

On 19:16 Tue 20 Aug     , Rob Herring wrote:
> On Tue, Aug 20, 2024 at 04:36:06PM +0200, Andrea della Porta wrote:
> > A missing or empty dma-ranges in a DT node implies a 1:1 mapping for dma
> > translations. In this specific case, rhe current behaviour is to zero out
> 
> typo

Fixed, thanks!

> 
> > the entire specifier so that the translation could be carried on as an
> > offset from zero.  This includes address specifier that has flags (e.g.
> > PCI ranges).
> > Once the flags portion has been zeroed, the translation chain is broken
> > since the mapping functions will check the upcoming address specifier
> 
> What does "upcoming address" mean?

Sorry for the confusion, this means "address specifier (with valid flags) fed
to the translating functions and for which we are looking for a translation".
While this address has some valid flags set, it will fail the translation step
since the ranges it is matched against have flags zeroed out by the 1:1 mapping
condition.

> 
> > against mismatching flags, always failing the 1:1 mapping and its entire
> > purpose of always succeeding.
> > Set to zero only the address portion while passing the flags through.
> 
> Can you point me to what the failing DT looks like. I'm puzzled how 
> things would have worked for anyone.
> 

The following is a simplified and lightly edited) version of the resulting DT
from RPi5:

 pci@0,0 {
	#address-cells = <0x03>;
	#size-cells = <0x02>;
	......
	device_type = "pci";
	compatible = "pci14e4,2712\0pciclass,060400\0pciclass,0604";
	ranges = <0x82000000 0x00 0x00   0x82000000 0x00 0x00   0x00 0x600000>;
	reg = <0x00 0x00 0x00   0x00 0x00>;

	......

	rp1@0 {
		#address-cells = <0x02>;
		#size-cells = <0x02>;
		compatible = "simple-bus";
		ranges = <0xc0 0x40000000   0x01 0x00 0x00   0x00 0x400000>;
		dma-ranges = <0x10 0x00   0x43000000 0x10 0x00   0x10 0x00>;
		......
	};
 };

The pci@0,0 bridge node is automatically created by virtue of
CONFIG_PCI_DYNAMIC_OF_NODES, and has no dma-ranges, hence it implies 1:1 dma
mappings (flags for this mapping are set to zero).  The rp1@0 node has
dma-ranges with flags set (0x43000000). Since 0x43000000 != 0x00 any translation
will fail.
Regarding why no one has really complained about that: AFAIK this could
very well be an unusual scenario that is arising now that we have real use
case for platform devices behind a PCI endpoint and devices populated
dynamically from dtb overlay.

Many thanks,
Andrea

> 
> > 
> > Signed-off-by: Andrea della Porta <[email-masked]>
> > ---
> >  drivers/of/address.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/of/address.c b/drivers/of/address.c
> > index d669ce25b5f9..5a6d55a67aa8 100644
> > --- a/drivers/of/address.c
> > +++ b/drivers/of/address.c
> > @@ -443,7 +443,8 @@ static int of_translate_one(struct device_node *parent, struct of_bus *bus,
> >  	}
> >  	if (ranges == NULL || rlen == 0) {
> >  		offset = of_read_number(addr, na);
> > -		memset(addr, 0, pna * 4);
> > +		/* copy the address while preserving the flags */
> > +		memset(addr + pbus->flag_cells, 0, (pna - pbus->flag_cells) * 4);
> >  		pr_debug("empty ranges; 1:1 translation\n");
> >  		goto finish;
> >  	}
> > -- 
> > 2.35.3
> >