Re: [PATCH 4/5] [SCSI] Do not use platform_bus as a parent

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sat, 2014-07-26 at 13:11 -0700, Greg Kroah-Hartman wrote:
> On Fri, Jul 25, 2014 at 07:46:56AM -0700, James Bottomley wrote:
> > On Fri, 2014-07-25 at 15:23 +0100, Pawel Moll wrote:
> > > The host devices without a parent were "forcefully adopted"
> > > by platform bus. This patch removes this assignment. In
> > > effect the dev_dev may be NULL now, which means ISA.
> > > 
> > > Cc: James E.J. Bottomley <JBottomley@parallels.com>
> > > Cc: linux-scsi@vger.kernel.org
> > > Signed-off-by: Pawel Moll <pawel.moll@arm.com>
> > > ---
> > > 
> > > This patch is a part of effort to remove references to platform_bus
> > > and make it static.
> > > 
> > > James, could you please have a look and advice if the change is
> > > correct? Would you happen to know the "real reasons" behind
> > > using the root platform_bus device a parent?
> > 
> > Yes, for DMA purposes, the parent cannot now be NULL; we'll get a panic
> > in the DMA transfers if it is.  A lot of the legacy ISA device on x86
> > and I thought some ARM SOC devices don't pass in the parent device, so
> > we hang them off a known parent.
> 
> The "generic" platform bus device is not a "known parent".  I don't
> understand the difference between just setting the parent to be NULL,
> which will then have a "proper" parent pointer filled in by the driver
> core when the device is registered, or faking it out here.  What is the
> difference?

If you set the parent to NULL, the host template dma_dev will end up
NULL as well and that will trigger a NULL deref panic in the dma segment
routines.

If you want to remove platform_bus, we have to have a well known device
to set dma_dev to at scsi_host_add time.

> In the end, the device always ends up with a parent pointer, right?

The parent pointer isn't the problem ... assigning the correct dma
device is.

James