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

[Marek Vasut <marek.vasut@gmail.com>]

On 10/16/19 8:12 PM, Rob Herring wrote:
> On Wed, Oct 16, 2019 at 11:18 AM Lorenzo Pieralisi
> <lorenzo.pieralisi@arm.com> wrote:
>>
>> [+RobH, Robin]
>>
>> On Wed, Oct 16, 2019 at 05:29:50PM +0200, Marek Vasut wrote:
>>
>> [...]
>>
>>>>> The firmware provides all the ranges which are available and usable,
>>>>> that's the hardware description and that should be in the DT.
>>>>
>>>> If the HW (given that those dma-ranges are declared for the PCI host
>>>> controller) can't be programmed to enable those DMA ranges - those
>>>> ranges are neither available nor usable, ergo DT is broken.
>>>
>>> The hardware can be programmed to enable those DMA ranges, just not all
>>> of them at the same time.
>>
>> Ok, we are down to DT bindings interpretation then.
>>
>>> It's not the job of the bootloader to guess which ranges might the next
>>> stage like best.
>>
>> By the time this series:
>>
>> https://patchwork.ozlabs.org/user/todo/linux-pci/?series=132419
>>
>> is merged, your policy will require the host controller driver to
>> remove the DMA ranges that could not be programmed in the inbound
>> address decoders from the dma_ranges list, otherwise things will
>> fall apart.
> 
> I don't think the above series has too much impact on this. It's my
> other series dealing with dma masks that's relevant because for dma
> masks we only ever look at the first dma-ranges entry. We either have
> to support multiple addresses and sizes per device (the only way to
> really support any possible dma-ranges), merge entries to single
> offset/mask or have some way to select which range entry to use.
> 
> So things are broken to some extent regardless unless MAX_NR_INBOUND_MAPS == 1.
> 
>>>>> 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 ?

-- 
Best regards,
Marek Vasut