Re: [PATCH 4/7] of: configure the platform device dma_mask and dma_pfn_offset

[Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Fri, Mar 7, 2014 at 10:02 AM, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org> wrote:
> On Friday 07 March 2014, Santosh Shilimkar wrote:
>> >> +
>> >> +       ret = dma_set_mask(dev, dma_mask);
>> >> +       if (ret < 0) {
>> >> +               dev_err(dev, "failed to set DMA mask %pad\n", &dma_mask);
>> >> +               dev->dma_mask = NULL;
>> >> +               return;
>> >> +       }
>> >> +
>> >> +       dev_dbg(dev, "dma_pfn_offset(%#08lx) dma_mask(%pad)\n",
>> >> +               dev->dma_pfn_offset, dev->dma_mask);
>> >> +
>> >> +       ret = dma_set_coherent_mask(dev, dma_mask);
>> >
>> > I think these 2 calls belong in the drivers, not here.
>> >
>> I also had same initial thought but Arnd mentioned that its a
>> shared responsibility between ARCH and drivers. Driver which
>> could be common between arches not always have the correct
>> mask information and it can change based on which arch it
>> is running.
>>
>> With some discussion back and forth, we thought updating
>> the dma_mask while the device getting created, would be
>> better place since we can find the arch capability at
>> this centralise code and update it.
>>
>> Ofcourse its bit debatable as the question you asked is
>> bit obvious as well. I let Arnd give his view here.
>
> If we set the mask *here*, we probably don't want to call 'dma_set_mask', but
> write to the mask directly, or we could call dma_coerce_mask_and_coherent(),
> which is really for overriding the mask pointer and value at once in cases
> where you absolutely know what it should be.
>
> We do need to decide what interface we want to use in platform device drivers,
> and I'm hoping that Russell has some idea which one he prefers:
>
> a) Follow what we do for PCI devices: assume that we can do DMA_MASK(32)
> on any device, and have drivers call dma_set_mask(DMA_MASK(64)) on devices
> that would like to do more than that, or call e.g. dma_set_mask(DMA_MASK(28))
> for devices that can do less than 32 bit, as given in the argument. This
> approach would be most consistent with the way PCI works, but it doesn't
> really work well for the case where the mask is less than 32-bit and the
> device driver doesn't know that.
>
> b) Never have to call dma_set_mask() for platform devices and assume that the
> platform code sets it up correctly. This would probably be the simpler
> solution, and I can't think of any downsides at the moment.

I don't think we want this. In the case of setting up 64-bit masters,
it is typically the device that knows if it can do 64-bit DMA either
thru a capabilities register or compatible value. That device specific
knowledge should really stay within the device's driver.

Rob

>
> In either case we probably want to call something like dt_dma_configure()
> from dma_set_mask() again to make sure that we stay within the limits
> imposed by the bus structure.
>
>         Arnd
>
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