Re: [PATCH v5 04/12] PCI: brcmstb: add dma-range mapping for inbound traffic

[Florian Fainelli <f.fainelli@gmail.com>]

On 9/24/2018 8:01 AM, Jim Quinlan wrote:
> On Mon, Sep 24, 2018 at 4:25 AM Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
>>
>> On Fri, 21 Sep 2018 at 19:41, Jim Quinlan <jim2101024@gmail.com> wrote:
>>>
>>> On Thu, Sep 20, 2018 at 5:39 PM Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>
>>>> On 09/20/2018 02:33 PM, Ard Biesheuvel wrote:
>>>>> On 20 September 2018 at 14:31, Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>>> On 09/20/2018 02:04 PM, Ard Biesheuvel wrote:
>>>>>>> On 20 September 2018 at 13:55, Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>>>>> On 09/19/2018 07:19 PM, Ard Biesheuvel wrote:
>>>>>>>>> On 19 September 2018 at 07:31, Jim Quinlan <jim2101024@gmail.com> wrote:
>>>>>>>>>> The Broadcom STB PCIe host controller is intimately related to the
>>>>>>>>>> memory subsystem.  This close relationship adds complexity to how cpu
>>>>>>>>>> system memory is mapped to PCIe memory.  Ideally, this mapping is an
>>>>>>>>>> identity mapping, or an identity mapping off by a constant.  Not so in
>>>>>>>>>> this case.
>>>>>>>>>>
>>>>>>>>>> Consider the Broadcom reference board BCM97445LCC_4X8 which has 6 GB
>>>>>>>>>> of system memory.  Here is how the PCIe controller maps the
>>>>>>>>>> system memory to PCIe memory:
>>>>>>>>>>
>>>>>>>>>>    memc0-a@[        0....3fffffff] <=> pci@[        0....3fffffff]
>>>>>>>>>>    memc0-b@[100000000...13fffffff] <=> pci@[ 40000000....7fffffff]
>>>>>>>>>>    memc1-a@[ 40000000....7fffffff] <=> pci@[ 80000000....bfffffff]
>>>>>>>>>>    memc1-b@[300000000...33fffffff] <=> pci@[ c0000000....ffffffff]
>>>>>>>>>>    memc2-a@[ 80000000....bfffffff] <=> pci@[100000000...13fffffff]
>>>>>>>>>>    memc2-b@[c00000000...c3fffffff] <=> pci@[140000000...17fffffff]
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> So is describing this as
>>>>>>>>>
>>>>>>>>> dma-ranges = <0x0 0x0 0x0 0x0 0x0 0x40000000>,
>>>>>>>>>               <0x0 0x40000000 0x1 0x0 0x0 0x40000000>,
>>>>>>>>>               <0x0 0x80000000 0x0 0x40000000 0x0 0x40000000>,
>>>>>>>>>               <0x0 0xc0000000 0x3 0x0 0x0 0x40000000>,
>>>>>>>>>               <0x1 0x0 0x0 0x80000000 0x0 0x40000000>,
>>>>>>>>>               <0x1 0x40000000 0x0 0xc0000000 0x0 0x40000000>;
>>>>>>>>>
>>>>>>>>> not working for you? I haven't tried this myself, but since DT permits
>>>>>>>>> describing the inbound mappings this way, we should fix the code if it
>>>>>>>>> doesn't work at the moment.
>>>>>>>>
>>>>>>>> You mean encoding the memory controller index in the first cell? If that
>>>>>>>> works, that's indeed a much cleaner solution, though is it standard
>>>>>>>> compliant in any form?
>>>>>>>
>>>>>>> No those are just memory addresses (although I may have screwed up the
>>>>>>> order). From Documentation/devicetree/booting-without-of.txt:
>>>>>>>
>>>>>>> """
>>>>>>> Optional property:
>>>>>>> - dma-ranges: <prop-encoded-array> encoded as arbitrary number of triplets of
>>>>>>>          (child-bus-address, parent-bus-address, length). Each triplet specified
>>>>>>>          describes a contiguous DMA address range.
>>>>>>> """
>>>>>>>
>>>>>>
>>>>>> Then I am confused by your comment, that's what this patch does, it adds
>>>>>> support for reading "dma-ranges" from Device Tree and setting up inbound
>>>>>> windows using that. The only caveat is that because the PCIe root
>>>>>> complex has some ties with the memory bus architecture it is connected
>>>>>> to (SCB in our case) there is still a requirement to know the
>>>>>> translation between a given physical address and its backing memory
>>>>>> controller/aperture.
>>>>>>
>>>>>
>>>>> Ah ok, apologies for the noise then.
>>>>>
>>>>> I was hoping that having working support for dma-ranges would remove
>>>>> the need for the special phys<->dma conversion routines.
>>>>
>>>> What you describe definitively works with platform devices, but I am not
>>>> sure this is working for PCIe devices, although, conceptually it should,
>>>> yes.
>>> Sorry for my delay in responding.  One problem is that
>>> of_dma_configure() only looks at the first dma-range given and then
>>> converts it to dev->dma_pfn_offset which is respected by the DMA API.
>>> However, we often have multiple dma-ranges, not just one.  This is the
>>> big issue.
>>>
>>
>> Given the recent attention to getting these APIs in shape, this may be
>> something Robin or Christoph may care to look into?
> 
> It looks like this has been brought up before in the "[RFC PATCH] of:
> Fix DMA configuration for non-DT masters" thread aka
> 
> https://lists.linuxfoundation.org/pipermail/iommu/2017-April/021325.html
> 
> In the thread "Oza Oza", a Broadcom coworker probably dealing with the
> same exact problem as I,  enumerates three problems.   #1 and #2 are
> the exact same ones I've just given: the "dma-ranges" prop of the RC
> DT node is "skipped", and of_dma_get_range() only considers the first
> entry in any "dma-ranges".

Robin, is that something that is expected or should the "dma-ranges" 
somehow propagate from host bridge down the PCIe end-point drivers?

> 
> Thanks, Jim
> 
>>
>> In any case, the description of dma-ranges should be in sync with the
>> way Linux interprets it, so this is either a documentation bug or a
>> DMA layer bug.
>>
>>> There is another issue with of_dma_configure() being invoked by the EP
>>> driver on "bridge->parent->of_node", which is our RC device,
>>> Of_dma_configure() calls of_dma_range() on the of_get_next_parent() of
>>> our RC's device node and this misses the dma-ranges property which is
>>> contained within the RC.  I think I could workaround this but there is
>>> no getting around the first problem.
>>>
>>
>> IIUC dma-ranges should be added to the parent bus of a device, which I
>> guess is slightly ambiguous for a root complex that incorporates a
>> host bridge.

Humm, why is that ambiguous for a host bridge/root complex?
-- 
Florian