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

[Marek Vasut <marek.vasut@gmail.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