Re: [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Marek Vasut <marek.vasut@gmail.com>,
	Rob Herring <robh@kernel.org>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	PCI <linux-pci@vger.kernel.org>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Wolfram Sang <wsa@the-dreams.de>,
	"open list:MEDIA DRIVERS FOR RENESAS - FCP" 
	<linux-renesas-soc@vger.kernel.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges
Date: Fri, 18 Oct 2019 12:17:38 +0200	[thread overview]
Message-ID: <CAMuHMdUf6uH_64R9hOgFgmoUYWk_LkCxy9RLzV0Je=TFr-G87w@mail.gmail.com> (raw)
In-Reply-To: <20191018100655.GA47056@e119886-lin.cambridge.arm.com>

Hi Andrew,

On Fri, Oct 18, 2019 at 12:07 PM Andrew Murray <andrew.murray@arm.com> wrote:
> On Thu, Oct 17, 2019 at 12:33:24AM +0200, Marek Vasut wrote:
> > On 10/17/19 12:26 AM, Rob Herring wrote:
> > [...]
> > >>>> You can have multiple non-continuous DRAM banks for example. And an
> > >>>> entry for SRAM optionally. Each DRAM bank and/or the SRAM should have a
> > >>>> separate dma-ranges entry, right ?
> > >>>
> > >>> Not necessarily. We really only want to define the minimum we have to.
> > >>> The ideal system is no dma-ranges. Is each bank at a different
> > >>> relative position compared to the CPU's view of the system. That would
> > >>> seem doubtful for just DRAM banks. Perhaps DRAM and SRAM could change.
> > >>
> > >> Is that a question ? Anyway, yes, there is a bit of DRAM below the 32bit
> > >> boundary and some more above the 32bit boundary. These two banks don't
> > >> need to be continuous. And then you could add the SRAM into the mix.
> > >
> > > Continuous is irrelevant. My question was in more specific terms is
> > > (bank1 addr - bank0 addr) different for CPU's view (i.e phys addr) vs.
> > > PCI host view (i.e. bus addr)? If not, then that is 1 translation and
> > > 1 dma-ranges entry.
> >
> > I don't think it's different in that aspect. Except the bus has this
> > 32bit limitation, where it only sees subset of the DRAM.
> >
> > Why should the DMA ranges incorrectly cover also the DRAM which is not
> > present ?
>
> I think this is where there is a difference in understanding.
>
> If I understand correctly, the job of the dma-ranges property isn't to
> describe *what* ranges the PCI device can access - it's there to describe
> *how*, i.e. the mapping between PCI and CPU-visible memory.
>
> The dma-ranges property is a side-effect of how the busses are wired up
> between the CPU and PCI controller - and so it doesn't matter what is or
> isn't on those buses.
>
> It's the job of other parts of the system to ensure that PCI devices are
> told the correct addresses to write to, e.g. the enumerating software
> referring to a valid CPU visible address correctly translated for the view
> of the PCI device, ATS etc. And any IOMMU to enforce that.

Yep, that's what I thought, too.

> It sounds like there is a 1:1 mapping between CPU and PCI - in which case
> there isn't a reason for a dma-ranges.

There's still the 32-bit limitation: PCI devices can access low 32-bit addresses
only.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds