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

[Robin Murphy <robin.murphy@arm.com>]

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.

> 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 :(

Robin.