Re: [PATCH V2] mmc: sdhci-acpi: Add 64-bit DMA support

[Adrian Hunter <adrian.hunter@intel.com>]

On 28/10/2014 5:08 p.m., Arnd Bergmann wrote:
> On Tuesday 28 October 2014 16:14:46 Adrian Hunter wrote:
>> On 28/10/2014 3:54 p.m., Arnd Bergmann wrote:
>>> On Tuesday 28 October 2014 13:41:30 Adrian Hunter wrote:
>>>> On 28/10/14 12:18, Arnd Bergmann wrote:
>>>>> On Tuesday 28 October 2014 12:05:30 Adrian Hunter wrote:
>>>>>> On 28/10/14 11:43, Arnd Bergmann wrote:
>>>>>>> On Tuesday 28 October 2014 10:37:20 Adrian Hunter wrote:
>>>>>>>>    static int sdhci_acpi_enable_dma(struct sdhci_host *host)
>>>>>>>>    {
>>>>>>>> -       return 0;
>>>>>>>> +       struct sdhci_acpi_host *c = sdhci_priv(host);
>>>>>>>> +       struct device *dev = &c->pdev->dev;
>>>>>>>> +       int err = -1;
>>>>>>>> +
>>>>>>>> +       if (c->dma_setup)
>>>>>>>> +               return 0;
>>>>>>>> +
>>>>>>>> +       if (host->flags & SDHCI_USE_64_BIT_DMA) {
>>>>>>>> +               if (host->quirks2 & SDHCI_QUIRK2_BROKEN_64_BIT_DMA) {
>>>>>>>> +                       host->flags &= ~SDHCI_USE_64_BIT_DMA;
>>>>>>>> +               } else {
>>>>>>>> +                       err = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64));
>>>>>>>> +                       if (err)
>>>>>>>> +                               dev_warn(dev, "Failed to set 64-bit DMA mask\n");
>>>>>>>> +               }
>>>>>>>> +       }
>>>>>>>> +
>>>>>>>> +       if (err)
>>>>>>>> +               err = dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32));
>>>>>>>> +
>>>>>>>> +       c->dma_setup = !err;
>>>>>>>> +
>>>>>>>> +       return err;
>>>>>>>>    }
>>>>>>>>
>>>>>>>
>>>>>>> I don't think it's worth a dev_warn() message (maybe dev_info), there is nothing
>>>>>>
>>>>>> It is worth a dev_warn because 32-bit DMA can allocate memory for bounce
>>>>>> buffers which jeopardizes memory reclaim.
>>>>>
>>>>> Then you should also warn if SDHCI_USE_64_BIT_DMA isn't or if
>>>>> SDHCI_QUIRK2_BROKEN_64_BIT_DMA is set I guess.
>>>>
>>>> The warning is for when the controller supports 64-bit, not when it doesn't.
>>>
>>> But why warn about a feature of the controller being present? You just
>>> said it's a problem for memory reclaim if 64-bit DMA is not supported.
>>
>> The warning is for when the controller supports 64-bit but it can't
>> get a 64-bit DMA mask, and might therefore need to bounce things.
>
> What does "can't get a 64-bit DMA mask" mean? This is just a different
> way to say it doesn't support 64-bit for some reason.

The host controller advertises whether it is capable of 64-bit DMA.  If
it is 64-bit capable but the driver cannot get a 64-bit DMA mask it issues
a warning.