Re: [PATCH] usb: xhci: inherit dma_mask from bus if set correctly

[Pankaj Dubey]

On Tue, 2 Apr 2019 at 15:34, Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 02/04/2019 10:40, Pankaj Dubey wrote:
> > From: Sriram Dash <sriram.dash@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > Signed-off-by: Sriram Dash <sriram.dash@xxxxxxxxxxx>
> > ---
> >   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);
> >