Re: [PATCH v3 02/16] mfd: mfd-core: Don't overwrite the dma_mask of the child device

[Andy Shevchenko]

On Tue, Apr 28, 2020 at 02:06:20PM +0100, Robin Murphy wrote:
> On 2020-04-28 1:45 pm, Andy Shevchenko wrote:
> > On Thu, Apr 23, 2020 at 07:45:29PM +0200, Michael Walle wrote:
> > > Commit cdfee5623290 ("driver core: initialize a default DMA mask for
> > > platform device") initialize the DMA of a platform device. But if the
> > > parent doesn't have a dma_mask set, for example if it's an I2C device,
> > > the dma_mask of the child platform device will be set to zero again.
> > > Which leads to many "DMA mask not set" warnings, if the MFD cell has the
> > > of_compatible property set.
> > 
> > I'm wondering why parent doesn't have it.
> 
> Because the parent isn't on a DMA-capable bus, and thus really shouldn't
> have a valid DMA configuration ever.

Then how come a child is DMA capable? MFD takes a physical device node as a
parent and creates one of several children with that device as a parent. DMA
mask is a property of the device which *does DMA*. Obviously a child is not
correct device for that.

Where am I mistaken?

> > I remember we have explicit patches in the past for buses such as PCI and AMBA
> > to set default DMA mask for all physical devices on the respective bus, of
> > course they can individually override it later.
> > 
> > So, this seems to me a paper over the real issue (absence of default DMA mask
> > where it's needed) and devices should explicitly define it if they disagree
> > with default.
> > 
> > If I'm wrong, you really need elaborate commit message much better.
> 
> The problem here is that MFD children are created as platform devices
> (regardless of what their parent is) and assigned an of_node, at which point
> they look pretty much indistinguishable from SoC devices created by the
> of_platform code, that *do* have to be assumed to be DMA-capable to prevent
> ~90% of existing devicetrees from breaking.
> 
> Of course the real fundamental issue is the platform bus itself, but it's
> way too late to fix that :(

I don't think it's an issue, rather in model you are describing. Or I miss
something not so obvious.

-- 
With Best Regards,
Andy Shevchenko