Date:   Wed, 24 Apr 2019 11:58:55 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Pankaj Dubey <pankaj.dubey@...sung.com>,
        Sriram Dash <sriram.dash@...sung.com>, mathias.nyman@...el.com
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        gregkh@...uxfoundation.org, Jingoo Han <jingoohan1@...il.com>,
        Krzysztof Kozlowski <krzk@...nel.org>, mgautam@...eaurora.org,
        felipe.balbi@...ux.intel.com
Subject: Re: [PATCH] usb: xhci: inherit dma_mask from bus if set correctly

On 24/04/2019 10:05, Pankaj Dubey wrote:
> 
> On 4/10/19 4:32 AM, Robin Murphy wrote:
>> On 2019-04-09 3:56 am, Sriram Dash wrote:
>>> On Tue, Apr 2, 2019 at 9:53 PM Pankaj Dubey
>>> <pankaj.dubey@...sung.com> wrote:
>>>>
>>>> On Tue, 2 Apr 2019 at 15:34, Robin Murphy <robin.murphy@....com> wrote:
>>>>>
>>>>> On 02/04/2019 10:40, Pankaj Dubey wrote:
>>>>>> From: Sriram Dash <sriram.dash@...sung.com>
>>>>>>
>>>>>> The xhci forcefully converts the dma_mask to either 64 or 32 and the
>>>>>> dma-mask set by the bus is somewhat ignored. If the platform  sets
>>>>>> the
>>>>>> correct dma_mask, then respect that.
>>>>>
>>>>> It's expected for dma_mask to be larger than bus_dma_mask if the
>>>>> latter
>>>>> is set - conceptually, the device mask represents what the device is
>>>>> inherently capable of, while the bus mask represents external
>>>>> interconnect restrictions which individual drivers should not have to
>>>>> care about. The DMA API backend should take care of combining the
>>>>> two to
>>>>> find the intersection.
>>>>
>>>> Thanks for the review.
>>>>
>>>> We are dealing here with the xhci platform which inherits the dma mask
>>>> of the parent, which is from a controller device.
>>>>
>>>> When the controller dma mask is set by the platform in DT, what we
>>>> observe is, its not getting inherited properly and the xhci bus is
>>>> forcing the dma address to be either 32 bit or 64 bit.
>>>>
>>>> In "drivers/usb/host/xhci-plat.c" we have dma_mask setting as below:
>>>>
>>>>    /* Try to set 64-bit DMA first */
>>>> if (WARN_ON(!sysdev->dma_mask))
>>>>        /* Platform did not initialize dma_mask */
>>>>        ret = dma_coerce_mask_and_coherent(sysdev, DMA_BIT_MASK(64));
>>>> else
>>>>        ret = dma_set_mask_and_coherent(sysdev, DMA_BIT_MASK(64));
>>>>
>>>> So even if the controller device has set the dma_mask as per it's
>>>> configuration in DT, xhci-plat.c will override it here in else part.
>>>>
>>>> Next it goes to "drivers/usb/host/xhci.c" file, here we have code as:
>>>>
>>>> /* Set dma_mask and coherent_dma_mask to 64-bits,
>>>>    * if xHC supports 64-bit addressing */
>>>> if (HCC_64BIT_ADDR(xhci->hcc_params) &&
>>>>                   !dma_set_mask(dev, DMA_BIT_MASK(64))) {
>>>>           xhci_dbg(xhci, "Enabling 64-bit DMA addresses.\n");
>>>>           dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
>>>> } else {
>>>>           /*
>>>>            * This is to avoid error in cases where a 32-bit USB
>>>>            * controller is used on a 64-bit capable system.
>>>>            */
>>>>           retval = dma_set_mask(dev, DMA_BIT_MASK(32));
>>>>           if (retval)
>>>>                   return retval;
>>>>           xhci_dbg(xhci, "Enabling 32-bit DMA addresses.\n");
>>>>           dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
>>>> }
>>>>
>>>> So xhci will force the dma_mask to either DMA_BIT_MASK(32) or
>>>> DMA_BIT_MASK(64), what if my device needs other than 32 bit or 64 bit
>>>> dma_mask.
>>>>
>>>> The bus_dma_mask was introduced for a case when the bus from a
>>>> device's dma interface may carry fewer address bits. But apparently,
>>>> it is the only mask which retains the original dma addressing from the
>>>> parent. So basically what we observe is currently there is no way we
>>>> can pass dma_mask greater than 32-bit, from DT via dev->dma_mask or
>>>> dev->coherent_dma_mask due to below logic in
>>>>
>>>> from "drivers/of/platform.c" we have
>>>> static struct platform_device *of_platform_device_create_pdata(
>>>>                                           struct device_node *np,
>>>>                                           const char *bus_id,
>>>>                                           void *platform_data,
>>>>                                           struct device *parent)
>>>> {
>>>>         struct platform_device *dev;
>>>>         ...
>>>>         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>>>>         if (!dev->dev.dma_mask)
>>>>                dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
>>>>     ...
>>>> }
>>>>
>>>> and then in of_dma_configure function in "drivers/of/device.c" we
>>>> have..
>>>>
>>>> mask = DMA_BIT_MASK(ilog2(dma_addr + size - 1) + 1); //This mask
>>>> computation is going fine and gets mask greater than 32-bit if defined
>>>> in DT
>>>> dev->coherent_dma_mask &= mask;  // Here the higher bit [63:32] will
>>>> get truncated as coherent_dma_mask is initialized to DMA_BIT_MASK(32)
>>>> in platform.c
>>>>
>>>> *dev->dma_mask &= mask; //Same here higher bits will get truncated
>>>> /* ...but only set bus mask if we found valid dma-ranges earlier */
>>>> if (!ret)
>>>> dev->bus_dma_mask = mask; //Only bus_dma_mask can carry the original
>>>> mask as specified in platform DT.
>>>>
>>>> To minimise the impact on existing code, we reused the bus_dma_mask
>>>> for finding the dma addressing bits.
>>>>
>>>> Or other way we may need to initialise dma_mask/coherent_dma_mask as
>>>> DMA_BIT_MASK(64) in "drivers/of/platform.c" and let all devices set
>>>> dma_mask via DT using "dma-ranges" property or respective platform
>>>> driver.
>>>>
>>>>> Are you seeing an actual problem here, and if so
>>>>> on which platform? (If the bus mask is set at all then it wouldn't
>>>>> seem
>>>>> to be the DT PCI issue that I'm still trying to fix).
>>>>>
>>>>
>>>>
>>>> We are facing this issue in one of the Samsung's upcoming SoC where we
>>>> need dma_mask greater than 32-bit.
>>>>
>>>> Thanks,
>>>> Pankaj
>>>>> Robin.
>>>>>
>>>>>> Signed-off-by: Pankaj Dubey <pankaj.dubey@...sung.com>
>>>>>> Signed-off-by: Sriram Dash <sriram.dash@...sung.com>
>>>>>> ---
>>>>>>     drivers/usb/host/xhci.c | 10 ++++++++++
>>>>>>     1 file changed, 10 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
>>>>>> index 005e659..55cf89e 100644
>>>>>> --- a/drivers/usb/host/xhci.c
>>>>>> +++ b/drivers/usb/host/xhci.c
>>>>>> @@ -5119,6 +5119,16 @@ int xhci_gen_setup(struct usb_hcd *hcd,
>>>>>> xhci_get_quirks_t get_quirks)
>>>>>>                 dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
>>>>>>         }
>>>>>>
>>>>>> +     /*
>>>>>> +      * A platform may require coherent masks other than 64/32
>>>>>> bit, and we
>>>>>> +      * should respect that. If the firmware has already
>>>>>> requested for a
>>>>>> +      * dma-range, we inherit the dma_mask presuming the platform
>>>>>> knows
>>>>>> +      * what it is doing.
>>>>>> +      */
>>>>>> +
>>>>>> +     if (dev->bus_dma_mask)
>>>>>> +             dma_set_mask_and_coherent(dev, dev->bus_dma_mask);
>>>>>> +
>>>>>>         xhci_dbg(xhci, "Calling HCD init\n");
>>>>>>         /* Initialize HCD and host controller data structures. */
>>>>>>         retval = xhci_init(hcd);
>>>>>>
>>>
>>> Hello Robin,
>>>
>>> Hope you found the crux of the matter. Any comments on the same?
>>
>> Sorry, I never received either of these replies - I've just happened
>> to notice this thread again by pure chance while looking at the
>> linux-usb patchwork for something else entirely, and managed to dredge
>> an mbox off lore.kernel.org to reply to. Mail is not my area of
>> expertise, but looking at the headers of the initial patch in my inbox
>> it seems that outlook.com is doing SPF negotiation with samsung.com,
>> so sending via gmail (as those replies appear to be) may be failing
>> that and getting silently discarded (they're not even in my spam
>> quarantine).
>>
>>  From the snippets of code quoted above I don't see anything obviously
>> wrong, but I'll take a closer look tomorrow. AFAICS though, if
>> dev->bus_dma_mask is set then dev is probably the appropriate device
>> for DMA, so I wouldn't expect a problem - XHCI is inherently a 64-bit
>> device, so its driver *should* be setting a 64-bit mask in this case.
>> To reiterate, what's the nature of the DMA issue? Do the mapping
>> operations fail, or do you actually see transfers going wrong due to
>> address truncation? Also, which arch is involved here - is it arm64
>> (as I seem to have assumed), or something else?
>>
>> Robin.
>>
> 
> Regarding issue in above code snippet, current code sets
> "dev->dev.coherent_dma_mask" as DMA_BIT_MASK(32) in platform.c
> (irrespective of architecture) and when we parse "dma-ranges" property
> and try to set coherent_dma_mask or dma_mask in of_dma_configure
> function in "drivers/of/device.c", even if "dma-ranges" specified in DT
> is more than 32-bit, 32-63 bits will be cleared to zero due to masking
> done in platform.c.
> 
> So effectively we are not able to set dma_mask or coherent_dma_mask
> larger than 32-bit mask.

For better or worse, that is the expected and intended behaviour for the 
default device masks. Drivers these days are expected to explicitly set 
their supported mask to replace the default, but there are still some 
remaining legacy assumptions that the default masks are 32-bit, so 
making them bigger risks subtle breakage, and that's why 
of_dma_configure() does the weird things it does.

> For the SoC concerned here is based on arm64, the USB IP (64 bit
> capable) is connected to a 36-bit Data bus and a 32-bit Control bus. The
> 36-bit Data bus is connected to an IOMMU which translates the address
> before they are passed on to other blocks. Here we have IOMMU is capable
> of 40-bit addressing. But as data bus is only capable of 36-bit, we need
> to ensure that IOMMU translates to address which does not exceed 36-bit.
> 
> Without this fix we are observing context fault from IOMMU.

Thanks for the clarification.

> Now, to get a workaround to this problem, we are inheriting the
> bus_dma_mask which is apparently the only one which contains the 36-bit
> bus mask.

If the bus mask is correctly set to 36 bits, but the DMA API 
implementation is failing to take that into account and giving you 
40-bit DMA addresses, that is a bug in the DMA API implementation, and 
it needs to be fixed in that DMA API code, not worked around in 
individual drivers.

Is this a 32-bit Arm system, by any chance?

Robin.

> Or as alternate solution we need to change coherent_dma_mask default
> mask as DMA_BIT_MASK(64) rather DMA_BIT_MASK(32) so that in
> of_dma_configure, the dma_mask/coherent_dma_mask get populated from
> "dma_ranges" DT property during device registration.
> 
> 

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