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

From: Marek Vasut <marek.vasut@gmail.com>
To: Robin Murphy <robin.murphy@arm.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Rob Herring <robh@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	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>
Subject: Re: [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges
Date: Fri, 18 Oct 2019 16:26:29 +0200	[thread overview]
Message-ID: <3f2f3868-676c-edc4-0de1-d42b63186128@gmail.com> (raw)
In-Reply-To: <ce7d16ab-31b8-0992-b1d7-24f4a652ce5f@arm.com>

On 10/18/19 2:53 PM, Robin Murphy wrote:
> On 18/10/2019 13:22, Marek Vasut wrote:
>> On 10/18/19 11:53 AM, Lorenzo Pieralisi wrote:
>>> On Thu, Oct 17, 2019 at 05:01:26PM +0200, Marek Vasut wrote:
>>>
>>> [...]
>>>
>>>>> Again, just handling the first N dma-ranges entries and ignoring the
>>>>> rest is not 'configure the controller correctly'.
>>>>
>>>> It's the best effort thing to do. It's well possible the next
>>>> generation
>>>> of the controller will have more windows, so could accommodate the
>>>> whole
>>>> list of ranges.
> 
> In the context of DT describing the platform that doesn't make any
> sense. It's like saying it's fine for U-Boot to also describe a bunch of
> non-existent CPUs just because future SoCs might have them. Just because
> the system would probably still boot doesn't mean it's right.

It's the exact opposite of what you just described -- the last release
of U-Boot currently populates a subset of the DMA ranges, not a
superset. The dma-ranges in the Linux DT currently are a superset of
available DRAM on the platform.

>>>> Thinking about this further, this patch should be OK either way, if
>>>> there is a DT which defines more DMA ranges than the controller can
>>>> handle, handling some is better than failing outright -- a PCI which
>>>> works with a subset of memory is better than PCI that does not work
>>>> at all.
>>>
>>> OK to sum it up, this patch is there to deal with u-boot adding multiple
>>> dma-ranges to DT.
>>
>> Yes, this patch was posted over two months ago, about the same time this
>> functionality was posted for inclusion in U-Boot. It made it into recent
>> U-Boot release, but there was no feedback on the Linux patch until
>> recently.
>>
>> U-Boot can be changed for the next release, assuming we agree on how it
>> should behave.
>>
>>> I still do not understand the benefit given that for
>>> DMA masks they are useless as Rob pointed out and ditto for inbound
>>> windows programming (given that AFAICS the PCI controller filters out
>>> any transaction that does not fall within its inbound windows by default
>>> so adding dma-ranges has the net effect of widening the DMA'able address
>>> space rather than limiting it).
>>>
>>> In short, what's the benefit of adding more dma-ranges regions to the
>>> DT (and consequently handling them in the kernel) ?
>>
>> The benefit is programming the controller inbound windows correctly.
>> But if there is a better way to do that, I am open to implement that.
>> Are there any suggestions / examples of that ?
> 
> The crucial thing is that once we improve the existing "dma-ranges"
> handling in the DMA layer such that it *does* consider multiple entries
> properly, platforms presenting ranges which don't actually exist will
> almost certainly start going wrong, and are either going to have to fix
> their broken bootloaders or try to make a case for platform-specific
> workarounds in core code.
Again, this is exactly the other way around, the dma-ranges populated by
U-Boot cover only existing DRAM. The single dma-range in Linux DT covers
even the holes without existing DRAM.

So even if the Linux dma-ranges handling changes, there should be no
problem.

[...]

-- 
Best regards,
Marek Vasut