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

[Arnd Bergmann <arnd@arndb.de>]

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. 

	Arnd