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

From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: Marek Vasut <marek.vasut@gmail.com>
Cc: linux-pci@vger.kernel.org,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Wolfram Sang <wsa@the-dreams.de>,
	linux-renesas-soc@vger.kernel.org, robh@kernel.org,
	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 17:18:46 +0100	[thread overview]
Message-ID: <20191016161846.GC7457@e121166-lin.cambridge.arm.com> (raw)
In-Reply-To: <75fb3519-80eb-fec2-d3eb-cc1b884fef25@gmail.com>

[+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.

> >> 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.

Lorenzo