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From: Rob Herring <robh@kernel.org>
To: Marek Vasut <marek.vasut@gmail.com>
Cc: 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: Wed, 16 Oct 2019 15:25:03 -0500
Message-ID: <CAL_Jsq+4uaFJzk5jUPw+KssZvnji0WDh+QcFMok99XXntEhNTQ@mail.gmail.com> (raw)
In-Reply-To: <c835701d-ff0e-f1b8-af16-fe53febe5519@gmail.com>

On Wed, Oct 16, 2019 at 1:18 PM Marek Vasut <marek.vasut@gmail.com> wrote:
>
> On 10/16/19 8:12 PM, Rob Herring wrote:
> > On Wed, Oct 16, 2019 at 11:18 AM Lorenzo Pieralisi
> > <lorenzo.pieralisi@arm.com> wrote:
> >>
> >> [+RobH, Robin]
> >>
> >> On Wed, Oct 16, 2019 at 05:29:50PM +0200, Marek Vasut wrote:
> >>
> >> [...]
> >>
> >>>>> The firmware provides all the ranges which are available and usable,
> >>>>> that's the hardware description and that should be in the DT.
> >>>>
> >>>> If the HW (given that those dma-ranges are declared for the PCI host
> >>>> controller) can't be programmed to enable those DMA ranges - those
> >>>> ranges are neither available nor usable, ergo DT is broken.
> >>>
> >>> The hardware can be programmed to enable those DMA ranges, just not all
> >>> of them at the same time.
> >>
> >> Ok, we are down to DT bindings interpretation then.
> >>
> >>> It's not the job of the bootloader to guess which ranges might the next
> >>> stage like best.
> >>
> >> By the time this series:
> >>
> >> https://patchwork.ozlabs.org/user/todo/linux-pci/?series=132419
> >>
> >> is merged, your policy will require the host controller driver to
> >> remove the DMA ranges that could not be programmed in the inbound
> >> address decoders from the dma_ranges list, otherwise things will
> >> fall apart.
> >
> > I don't think the above series has too much impact on this. It's my
> > other series dealing with dma masks that's relevant because for dma
> > masks we only ever look at the first dma-ranges entry. We either have
> > to support multiple addresses and sizes per device (the only way to
> > really support any possible dma-ranges), merge entries to single
> > offset/mask or have some way to select which range entry to use.
> >
> > So things are broken to some extent regardless unless MAX_NR_INBOUND_MAPS == 1.
> >
> >>>>> The firmware cannot decide the policy for the next stage (Linux in
> >>>>> this case) on which ranges are better to use for Linux and which are
> >>>>> less good. Linux can then decide which ranges are best suited for it
> >>>>> and ignore the other ones.
> >>>>
> >>>> dma-ranges is a property that is used by other kernel subsystems eg
> >>>> IOMMU other than the RCAR host controller driver. The policy, provided
> >>>> there is one should be shared across them. You can't leave a PCI
> >>>> host controller half-programmed and expect other subsystems (that
> >>>> *expect* those ranges to be DMA'ble) to work.
> >>>>
> >>>> I reiterate my point: if firmware is broken it is better to fail
> >>>> the probe rather than limp on hoping that things will keep on
> >>>> working.
> >>>
> >>> But the firmware is not broken ?
> >>
> >> See above, it depends on how the dma-ranges property is interpreted,
> >> hopefully we can reach consensus in this thread, I won't merge a patch
> >> that can backfire later unless we all agree that what it does is
> >> correct.
> >
> > Defining more dma-ranges entries than the h/w has inbound windows for
> > sounds like a broken DT to me.
> >
> > What exactly does dma-ranges contain in this case? I'm not really
> > visualizing how different clients would pick different dma-ranges
> > entries.
>
> 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.

I suppose if your intent is to use inbound windows as a poor man's
IOMMU to prevent accesses to the holes, then yes you would list them
out. But I think that's wrong and difficult to maintain. You'd also
need to deal with reserved-memory regions too.

Rob

  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 17:57 [PATCH V3 1/3] PCI: rcar: Move the inbound index check marek.vasut
2019-08-09 17:57 ` [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges marek.vasut
2019-08-16 13:23   ` Simon Horman
2019-08-16 13:28     ` Marek Vasut
2019-08-16 13:38       ` Simon Horman
2019-08-16 17:41         ` Marek Vasut
2019-10-21 10:18       ` Andrew Murray
2019-10-26 18:03         ` Marek Vasut
2019-10-26 20:36           ` Andrew Murray
2019-10-26 21:06             ` Andrew Murray
2019-11-06 23:37             ` Marek Vasut
2019-11-07 14:19               ` Andrew Murray
2019-10-16 15:00   ` Lorenzo Pieralisi
2019-10-16 15:10     ` Marek Vasut
2019-10-16 15:26       ` Lorenzo Pieralisi
2019-10-16 15:29         ` Marek Vasut
2019-10-16 16:18           ` Lorenzo Pieralisi
2019-10-16 18:12             ` Rob Herring
2019-10-16 18:17               ` Marek Vasut
2019-10-16 20:25                 ` Rob Herring [this message]
2019-10-16 21:15                   ` Marek Vasut
2019-10-16 22:26                     ` Rob Herring
2019-10-16 22:33                       ` Marek Vasut
2019-10-17  7:06                         ` Geert Uytterhoeven
2019-10-17 10:55                           ` Marek Vasut
2019-10-17 13:06                             ` Robin Murphy
2019-10-17 14:00                               ` Marek Vasut
2019-10-17 14:36                                 ` Rob Herring
2019-10-17 15:01                                   ` Marek Vasut
2019-10-18  9:53                                     ` Lorenzo Pieralisi
2019-10-18 12:22                                       ` Marek Vasut
2019-10-18 12:53                                         ` Robin Murphy
2019-10-18 14:26                                           ` Marek Vasut
2019-10-18 15:44                                             ` Robin Murphy
2019-10-18 16:44                                               ` Marek Vasut
2019-10-18 17:35                                                 ` Robin Murphy
2019-10-18 18:44                                                   ` Marek Vasut
2019-10-21  8:32                                                     ` Geert Uytterhoeven
2019-10-18 10:06                         ` Andrew Murray
2019-10-18 10:17                           ` Geert Uytterhoeven
2019-10-18 11:40                             ` Andrew Murray
2019-08-09 17:57 ` [PATCH V3 3/3] PCI: rcar: Recalculate inbound range alignment for each controller entry marek.vasut
2019-10-21 10:39   ` Andrew Murray
2019-08-16 10:52 ` [PATCH V3 1/3] PCI: rcar: Move the inbound index check Lorenzo Pieralisi
2019-08-16 10:59   ` Marek Vasut
2019-08-16 11:10     ` Lorenzo Pieralisi
2019-10-15 20:14 ` Marek Vasut
2019-10-21 10:11 ` Andrew Murray

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