Re: [PATCH 1/2] [RFC] ata: ahci: Respect bus DMA constraints

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

On 3/18/19 2:14 PM, Robin Murphy wrote:
> On 17/03/2019 23:36, Marek Vasut wrote:
>> On 3/17/19 11:29 AM, Geert Uytterhoeven wrote:
>>> Hi Marek,
>>
>> Hi,
>>
>>> On Sun, Mar 17, 2019 at 12:04 AM Marek Vasut <marek.vasut@gmail.com>
>>> wrote:
>>>> On 3/16/19 10:25 PM, Marek Vasut wrote:
>>>>> On 3/13/19 7:30 PM, Christoph Hellwig wrote:
>>>>>> On Sat, Mar 09, 2019 at 12:23:15AM +0100, Marek Vasut wrote:
>>>>>>> On 3/8/19 8:18 AM, Christoph Hellwig wrote:
>>>>>>>> On Thu, Mar 07, 2019 at 12:14:06PM +0100, Marek Vasut wrote:
>>>>>>>>>> Right, but whoever *interprets* the device masks after the
>>>>>>>>>> driver has
>>>>>>>>>> overridden them should be taking the (smaller) bus mask into
>>>>>>>>>> account as
>>>>>>>>>> well, so the question is where is *that* not being done
>>>>>>>>>> correctly?
>>>>>>>>>
>>>>>>>>> Do you have a hint where I should look for that ?
>>>>>>>>
>>>>>>>> If this a 32-bit ARM platform it might the complete lack of support
>>>>>>>> for bus_dma_mask in arch/arm/mm/dma-mapping.c..
>>>>>>>
>>>>>>> It's an ARM 64bit platform, just the PCIe controller is limited
>>>>>>> to 32bit
>>>>>>> address range, so the devices on the PCIe bus cannot read the host's
>>>>>>> DRAM above the 32bit limit.
>>>>>>
>>>>>> arm64 should take the mask into account both for the swiotlb and
>>>>>> iommu case.  What are the exact symptoms you see?
>>>>>
>>>>> With the nvme, the device is recognized, but cannot be used.
>>>>> It boils down to PCI BAR access being possible, since that's all below
>>>>> the 32bit boundary, but when the device tries to do any sort of DMA,
>>>>> that transfer returns nonsense data.
>>>>>
>>>>> But when I call dma_set_mask_and_coherent(dev->dev,
>>>>> DMA_BIT_MASK(32) in
>>>>> the affected driver (thus far I tried this nvme, xhci-pci and ahci-pci
>>>>> drivers), it all starts to work fine.
>>>>>
>>>>> Could it be that the driver overwrites the (coherent_)dma_mask and
>>>>> that's why the swiotlb/iommu code cannot take this into account ?
>>>>>
>>>>>> Does it involve
>>>>>> swiotlb not kicking in, or iommu issues?
>>>>>
>>>>> How can I check ? I added printks into arch/arm64/mm/dma-mapping.c and
>>>>> drivers/iommu/dma-iommu.c , but I suspect I need to look elsewhere.
>>>>
>>>> Digging further ...
>>>>
>>>> drivers/nvme/host/pci.c nvme_map_data() calls dma_map_sg_attrs() and
>>>> the
>>>> resulting sglist contains entry with >32bit PA. This is because
>>>> dma_map_sg_attrs() calls dma_direct_map_sg(), which in turn calls
>>>> dma_direct_map_sg(), then dma_direct_map_page() and that's where it
>>>> goes
>>>> weird.
>>>>
>>>> dma_direct_map_page() does a dma_direct_possible() check before
>>>> triggering swiotlb_map(). The check succeeds, so the later isn't
>>>> executed.
>>>>
>>>> dma_direct_possible() calls dma_capable() with dev->dma_mask =
>>>> DMA_BIT_MASK(64) and dev->dma_bus_mask = 0, so
>>>> min_not_zero(*dev->dma_mask, dev->bus_dma_mask) returns
>>>> DMA_BIT_MASK(64).
>>>>
>>>> Surely enough, if I hack dma_direct_possible() to return 0,
>>>> swiotlb_map() kicks in and the nvme driver starts working fine.
>>>>
>>>> I presume the question here is, why is dev->bus_dma_mask = 0 ?
>>>
>>> Because that's the default, and almost no code overrides that?
>>
>> But shouldn't drivers/of/device.c set that for the PCIe controller ?
> 
> Urgh, I really should have spotted the significance of "NVMe", but
> somehow it failed to click :(

Good thing it did now :-)

> Of course the existing code works fine for everything *except* PCI
> devices on DT-based systems... That's because of_dma_get_range() has
> never been made to work correctly with the trick we play of passing the
> host bridge of_node through of_dma_configure(). I've got at least 2 or 3
> half-finished attempts at improving that, but they keep getting
> sidetracked into trying to clean up the various new of_dma_configure()
> hacks I find in drivers and/or falling down the rabbit-hole of starting
> to redesign the whole dma_pfn_offset machinery entirely. Let me dig one
> up and try to constrain it to solve just this most common "one single
> limited range" condition for the sake of making actual progress...

That'd be nice, thank you. I'm happy to test it on various devices here.

-- 
Best regards,
Marek Vasut