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

From: Marek Vasut <marek.vasut@gmail.com>
To: Rob Herring <robh@kernel.org>
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 23:15:19 +0200	[thread overview]
Message-ID: <88099c4f-4fb4-626e-f66f-3eb8861dfb2c@gmail.com> (raw)
In-Reply-To: <CAL_Jsq+4uaFJzk5jUPw+KssZvnji0WDh+QcFMok99XXntEhNTQ@mail.gmail.com>

On 10/16/19 10:25 PM, Rob Herring wrote:
[...]
>>>>>>> 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.

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.

> 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.

What's the problem with that? The bootloader has all that information
and can patch the DT correctly. In fact, in my specific case, I have
platform which can be populated with differently sized DRAM, so the
holes are also dynamically calculated ; there is no one DT then, the
bootloader is responsible to generate the dma-ranges accordingly.

-- 
Best regards,
Marek Vasut