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From: Marek Vasut <marek.vasut@gmail.com>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Simon Horman <horms@verge.net.au>,
	linux-pci@vger.kernel.org,
	Marek Vasut <marek.vasut+renesas@gmail.com>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Wolfram Sang <wsa@the-dreams.de>,
	linux-renesas-soc@vger.kernel.org, Robin.Murphy@arm.com
Subject: Re: [PATCH V3 2/3] PCI: rcar: Do not abort on too many inbound dma-ranges
Date: Sat, 16 Nov 2019 16:48:45 +0100
Message-ID: <3424b83c-4693-0259-ac3d-ea10a3f98377@gmail.com> (raw)
In-Reply-To: <20191107141906.GB43905@e119886-lin.cambridge.arm.com>

On 11/7/19 3:19 PM, Andrew Murray wrote:
> On Thu, Nov 07, 2019 at 12:37:44AM +0100, Marek Vasut wrote:
>> On 10/26/19 10:36 PM, Andrew Murray wrote:
>> [...]>> But this still leaves me with one open question -- how do I
>> figure out
>>>> what to program into the PCI controller inbound windows, so that the
>>>> controller correctly filters inbound transfers which are targetting
>>>> nonexisting memory ?
>>>
>>> Your driver should program into the RC->CPU windows, the exact ranges
>>> described in the dma-ranges. Whilst also respecting the alignment and
>>> max-size rules your controller has (e.g. the existing upstream logic
>>> and also the new logic that recalculates the alignment per entry).
>>>
>>> As far as I can tell from looking at your U-Boot patch, I think I'd expect
>>> a single dma-range to be presented in the DT, that describes
>>> 0:0xFFFFFFFF => 0:0xFFFFFFFF. This is because 1) I understand your
>>> controller is limited to 32 bits. And 2) there is a linear mapping between
>>> PCI and CPU addresses (given that the second and third arguments on
>>> pci_set_region are both the same).
>>>
>>> As you point out, this range includes lots of things that you don't
>>> want the RC to touch - such as non-existent memory. This is OK, when
>>> Linux programs addresses into the various EP's for them to DMA to host
>>> memory, it uses its own logic to select addresses that are in RAM, the
>>> purpose of the dma-range is to describe what the CPU RAM address looks
>>> like from the perspective of the RC (for example if the RC was wired
>>> with an offset such that made memory writes from the RC made to
>>> 0x00000000 end up on the system map at 0x80000000, we need to tell Linux
>>> about this offset. Otherwise when a EP device driver programs a DMA
>>> address of a RAM buffer at 0x90000000, it'll end up targetting
>>> 0x110000000. Thankfully our dma-range will tell Linux to apply an offset
>>> such that the actual address written to the EP is 0x10000000.).
>>
>> I understand that Linux programs the endpoints correctly. However this
>> still doesn't prevent the endpoint from being broken and from sending a
>> transaction to that non-existent memory.
> 
> Correct.
> 
>> The PCI controller can prevent
>> that and in an automotive SoC, I would very much like the PCI controller
>> to do just that, rather than hope that the endpoint would always work.
> 
> OK I understand - At least when working on the assumption that your RC will
> block RC->CPU transactions that are not described in any of it's windows.
> Thus you want to use the dma-ranges as a means to configure your controller
> to do this.

Yes

> What actually happens if you have a broken endpoint that reads/writes to
> non-existent memory on this hardware? Ideally the RC would generate a
> CA or UR back to the endpoint - does something else happen? Lockup, dead RC,
> performance issues?

The behavior is undefined.

> Using built-in features of the RC to prevent it from sending transactions
> to non-existent addresses is clearly helpful. But of course it doesn't stop
> a broken EP from writing to existent addresses, so only provides limited
> protection.

Correct.

> Despite the good intentions here, it doesn't seem like dma-ranges is
> designed for this purpose and as the hardware has limited ranges it will
> only be best-effort.
So what other options do we have ?

-- 
Best regards,
Marek Vasut

  reply index

Thread overview: 51+ 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
2019-11-16 15:48                 ` Marek Vasut [this message]
2019-11-18 18:42                   ` Robin Murphy
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
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
2019-11-19 12:10                                                     ` Robin Murphy
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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