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

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

On 28/10/14 17:38, Arnd Bergmann wrote:
> On Tuesday 28 October 2014 17:14:25 Adrian Hunter wrote:
>> 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.
> 
> But the host controller doesn't know if it's 64-bit capable, nor should
> it know. All the host controller knows is that it has registers to perform
> 64-bit DMA, but the platform code (ACPI in your case) is the only thing that
> knows how many of those address lines are connected to an upstream bus.

No, the SDHCI spec. says:

	64-bit System Bus Support
	Setting 1 to this bit indicates that the Host Controller supports 64-bit
	address descriptor mode and is connected to 64-bit address system bus.

So the expectation is that it is on a 64-bit bus.  The warning is reasonable.