Re: [PATCH v7 1/2] usb: make xhci platform driver use 64 bit or 32 bit DMA

[Mathias Nyman]

On 31.08.2015 21:58, Duc Dang wrote:
> On Thu, Aug 20, 2015 at 12:38 PM, Duc Dang <dhdang@xxxxxxx> wrote:
> > The xhci platform driver needs to work on systems that
> > either only support 64-bit DMA or only support 32-bit DMA.
> > Attempt to set a coherent dma mask for 64-bit DMA, and
> > attempt again with 32-bit DMA if that fails.
> >
> > [dhdang: regenerate the patch over 4.2-rc5 and address new comments]
> > Signed-off-by: Mark Langsdorf <mlangsdo@xxxxxxxxxx>
> > Tested-by: Mark Salter <msalter@xxxxxxxxxx>
> > Signed-off-by: Duc Dang <dhdang@xxxxxxx>
> >
> > ---
> > Changes from v6:
> >          -Add WARN_ON if dma_mask is NULL
> >          -Use dma_coerce_mask_and_coherent to assign
> >          dma_mask and coherent_dma_mask
> >
> > Changes from v5:
> >          -Change comment
> >          -Assign dma_mask to coherent_dma_mask if dma_mask is NULL
> >          to make sure dma_set_mask_and_coherent does not fail prematurely.
> >
> > Changes from v4:
> >          -None
> >
> > Changes from v3:
> >          -Re-generate the patch over 4.2-rc5
> >          -No code change.
> >
> > Changes from v2:
> >          -None
> >
> > Changes from v1:
> >          -Consolidated to use dma_set_mask_and_coherent
> >          -Got rid of the check against sizeof(dma_addr_t)
> >
> >   drivers/usb/host/xhci-plat.c | 21 +++++++++++++--------
> >   1 file changed, 13 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
> > index 890ad9d..e4c7f9d 100644
> > --- a/drivers/usb/host/xhci-plat.c
> > +++ b/drivers/usb/host/xhci-plat.c
> > @@ -93,14 +93,19 @@ static int xhci_plat_probe(struct platform_device *pdev)
> >          if (irq < 0)
> >                  return -ENODEV;
> >
> > -       /* Initialize dma_mask and coherent_dma_mask to 32-bits */
> > -       ret = dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32));
> > -       if (ret)
> > -               return ret;
> > -       if (!pdev->dev.dma_mask)
> > -               pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
> > -       else
> > -               dma_set_mask(&pdev->dev, DMA_BIT_MASK(32));
> > +       /* Throw a waring if broken platform code didn't initialize dma_mask */
> > +       WARN_ON(!pdev->dev.dma_mask);
> > +       /*
> > +        * Try setting dma_mask and coherent_dma_mask to 64 bits,
> > +        * then try 32 bits
> > +        */
> > +       ret = dma_coerce_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
> > +       if (ret) {
> > +               ret = dma_coerce_mask_and_coherent(&pdev->dev,
> > +                                                  DMA_BIT_MASK(32));
> > +               if (ret)
> > +                       return ret;
> > +       }
> >
> >          hcd = usb_create_hcd(driver, &pdev->dev, dev_name(&pdev->dev));
> >          if (!hcd)
> > --
> > 1.9.1
>
> Hi Greg, Arnd, Russell,
>
> Do you have any more comment about this patch set?

I'm not sure I fully understand why we need to try the 64 bit DMA mask in platform probe.

As I understood it we just want to have some DMA mask set before calling usb_create_hcd()
to make sure USB core gets the "uses_dma" flag set and dma_set_mask() won't fail because of
missing dev->dma_mask.

The correct DMA mask is set later in xhci_gen_setup()

We also need to make sure the controller supports 64bit addressing capability before setting a 64 bit DMA mask.
(bit 0 in HCCPARAMS)

So for platform devices it goes look go this:

xhci_plat_probe()
  usb_create_hcd()
    usb_create_shared_hcd()
      hcd->self.uses_dma = (dev->dma_mask != NULL);
  usb_add_hcd()
    hcd->driver->reset()  (.reset = xhci_plat_setup)
      xhci_plat_setup()
        xhci_gen_setup()
          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));
          }

or do we end up with dev->dma_mask = NULL on 64-bit DMA only after trying to set it to 32 in xhci_plat_probe(),
and thus also failing the dma_set_mask() in xhci_gen_setup()?

-Mathias










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