Re: [PATCH 1/2] of: device: Allow DMA range map to be set before of_dma_configure_id

[Paul Kocialkowski <paul.kocialkowski@bootlin.com>]

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Hi Robin,

On Mon 18 Jan 21, 13:27, Robin Murphy wrote:
> On 2021-01-16 17:07, Paul Kocialkowski wrote:
> > Hi Robin,
> > 
> > Le Sat 16 Jan 21, 14:57, Robin Murphy a écrit :
> > > On 2021-01-15 17:58, Paul Kocialkowski wrote:
> > > > A mechanism was recently introduced for the sunxi architecture where
> > > > the DMA offset for specific devices (under the MBUS) is set by a common
> > > > driver (sunxi_mbus). This driver calls dma_direct_set_offset to set
> > > > the device's dma_range_map manually.
> > > > 
> > > > However this information was overwritten by of_dma_configure_id, which
> > > > obtains the map from of_dma_get_range (or keeps it NULL when it fails
> > > > and the force_dma argument is true, which is the case for platform
> > > > devices).
> > > > 
> > > > As a result, the dma_range_map was always overwritten and the mechanism
> > > > could not correctly take effect.
> > > > 
> > > > This adds a check to ensure that no previous DMA range map is
> > > > overwritten and prints a warning when the map was already set while
> > > > also being available from dt. In this case, the map that was already
> > > > set is kept.
> > > 
> > > Hang on, the hard-coded offset is only intended to be installed when there
> > > *isn't* anything described in DT, in which case of_dma_get_range() should
> > > always bail out early without touching it anyway. This sounds like
> > > something's not quite right in the MBUS driver, so I don't think working
> > > around it in core code is really the right thing to do.
> > 
> > That's right, there is no practical case where the two are in conflict.
> > The problem that I'm solving here is that dev->dma_range_map is *always*
> > assigned, even when of_dma_get_range bailed and map still is NULL.
> > 
> > This has the effect of always overwriting what the MBUS driver does
> > (and leaking its memory too).
> 
> Oh, I should have looked closer at of_dma_configure_id() itself. I was
> assuming that b4bdc4fbf8d0 had been tested and at least *could* work, but
> apparently not :/
> 
> Indeed it seems there was a fundamental oversight in e0d072782c73
> ("dma-mapping: introduce DMA range map, supplanting dma_pfn_offset"),
> equivalent to the same one I previously made myself with bus_dma_mask and
> fixed in 6778be4e5209 ("of/device: Really only set bus DMA mask when
> appropriate"). I think same simpler fix is the right one for this case too,
> i.e. just move the assignment to dev->dma_range_map up under the "if (!ret)"
> condition. Assigning it this late - after the IOMMU and arch setup - looks
> wrong anyway, even if it isn't currently an issue in practice (in principle
> an IOMMU driver *could* start trying to figure out reserved regions around
> DMA windows for a device as early as its .of_xlate callback, even though
> that's not the intent of the API design).

Okay sounds good, I'll resend a patch with the assignment under if (!ret)!

> Luckily dma_range_map hasn't been hooked up in the equivalent ACPI path yet,
> so you're off the hook for fixing that too :)

Good for me :)

Cheers,

Paul

> > > Do you have a case where one of the relevant devices inherits a "dma-ranges"
> > > via the regular hierarchy without indirecting via an "interconnects"
> > > reference? Currently you're only checking for the latter, so that would be
> > > one way things could go awry (although to be a problem, said "dma-ranges"
> > > would also have to encode something *other* than the appropriate MBUS
> > > offset, which implies an incorrect or at least inaccurately-structured DT as
> > > well).
> > 
> > No, I think things are good in that regard. No messed up dt or anything like
> > that :)
> > 
> > Cheers,
> > 
> > Paul
> > 
> > > Robin.
> > > 
> > > > Fixes: b4bdc4fbf8d0 ("soc: sunxi: Deal with the MBUS DMA offsets in a central place")
> > > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > > > ---
> > > >    drivers/of/device.c | 9 ++++++++-
> > > >    1 file changed, 8 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/of/device.c b/drivers/of/device.c
> > > > index aedfaaafd3e7..db1b8634c2c7 100644
> > > > --- a/drivers/of/device.c
> > > > +++ b/drivers/of/device.c
> > > > @@ -181,7 +181,14 @@ int of_dma_configure_id(struct device *dev, struct device_node *np,
> > > >    	arch_setup_dma_ops(dev, dma_start, size, iommu, coherent);
> > > > -	dev->dma_range_map = map;
> > > > +	if (!dev->dma_range_map) {
> > > > +		dev->dma_range_map = map;
> > > > +	} else if (map) {
> > > > +		dev_warn(dev,
> > > > +			 "DMA range map was already set, ignoring range map from dt\n");
> > > > +		kfree(map);
> > > > +	}
> > > > +
> > > >    	return 0;
> > > >    }
> > > >    EXPORT_SYMBOL_GPL(of_dma_configure_id);
> > > > 
> > 

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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