Re: [PATCH v3 02/16] mfd: mfd-core: Don't overwrite the dma_mask of the child device
From: Robin Murphy
Date: Tue Apr 28 2020 - 09:06:29 EST
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.