[PATCH v5 1/2] usb: make xhci platform driver use 64 bit or 32 bit DMA

[Duc Dang]

On Sat, Aug 8, 2015 at 2:22 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Fri, Aug 07, 2015 at 08:18:48PM -0700, Duc Dang wrote:
>> diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
>> index 890ad9d..5d03f8b 100644
>> --- a/drivers/usb/host/xhci-plat.c
>> +++ b/drivers/usb/host/xhci-plat.c
>> @@ -93,14 +93,14 @@ 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));
>> +     /* Try setting the coherent_dma_mask to 64 bits, then try 32 bits */
>> +     ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(64));
>> +     if (ret) {
>> +             ret = dma_set_mask_and_coherent(&pdev->dev, DMA_BIT_MASK(32));
>> +             if (ret)
>> +                     return ret;
>> +     }
>
> Note that dma_set_mask_and_coherent() and the original code are not
> equivalent because of this:
>
>         if (!pdev->dev.dma_mask)
>                 pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
>
> If we know that pdev->dev.dma_mask will always be initialised at this
> point, then the above change is fine.  If not, it's introducing a
> regression - dma_set_mask_and_coherent() will fail if pdev->dev.dma_mask
> is NULL (depending on the architectures implementation of dma_set_mask()).
>
> Prefixing the above change with the two lines I mention above would
> ensure equivalent behaviour.  Even if we do want to get rid of this,
> I'd advise to do it as a separate patch after this change, which can
> be independently reverted if there's problems with its removal.
>
Hi Russell,

I will add the 2 lines you mentioned back to next version of the
patch. It is safer to do it that way as I do not see
pdev->dev.dma_mask gets initialized before the call
dma_set_mask_and_coherent  inside this xhci_plat.c file.

> --
> FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
> according to speedtest.net.

-- 
Regards,
Duc Dang.