[4/5,SCSI] Do not use platform_bus as a parent

Commit Message

Greg KH July 27, 2014, 3:07 p.m. UTC

On Sun, Jul 27, 2014 at 07:52:57AM +0400, James Bottomley wrote:
> 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 <[e-mail]>
> > > > Cc: [e-mail]
> > > > Signed-off-by: Pawel Moll <[e-mail]>
> > > > ---
> > > > 
> > > > 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.

Why not set the dma_dev after you call device_add()?  That way you will
pick up the right parent no matter what.

> > 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.

Ah, ok, it's a scsi core thing, not a driver core thing, that's less
confusing now.  For a "fallback" of a platform device, if you switch the
lines around you should be fine, something like this patch perhaps:

Comments

Pawel Moll Aug. 1, 2014, 5:25 p.m. UTC | #1

On Sun, 2014-07-27 at 16:07 +0100, Greg Kroah-Hartman wrote:
> Ah, ok, it's a scsi core thing, not a driver core thing, that's less
> confusing now.  For a "fallback" of a platform device, if you switch the
> lines around you should be fine, something like this patch perhaps:
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index 3cbb57a8b846..d8d3b294f5bc 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -218,16 +218,16 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
>  		goto fail;
>  
>  	if (!shost->shost_gendev.parent)
> -		shost->shost_gendev.parent = dev ? dev : &platform_bus;
> -	if (!dma_dev)
> -		dma_dev = shost->shost_gendev.parent;
> -
> -	shost->dma_dev = dma_dev;
> +		shost->shost_gendev.parent = dev;
>  
>  	error = device_add(&shost->shost_gendev);
>  	if (error)
>  		goto out;
>  
> +	if (!dma_dev)
> +		dma_dev = shost->shost_gendev.parent;
> +	shost->dma_dev = dma_dev;
> +
>  	pm_runtime_set_active(&shost->shost_gendev);
>  	pm_runtime_enable(&shost->shost_gendev);
>  	device_enable_async_suspend(&shost->shost_gendev);

But this will still make shost->dma_dev == NULL in the cases James
quoted:

On Fri, 2014-07-25 at 15:46 +0100, James Bottomley wrote:
> 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.
> 
> You can grep for it; these are the devices that will begin to panic if
> you apply this patch:
> 
> arch/ia64/hp/sim/simscsi.c:     error = scsi_add_host(host, NULL);
> drivers/scsi/a2091.c:   error = scsi_add_host(instance, NULL);
> drivers/scsi/a3000.c:   error = scsi_add_host(instance, NULL);
> drivers/scsi/aha152x.c: if( scsi_add_host(shpnt, NULL) ) {
> drivers/scsi/gdth.c:    error = scsi_add_host(shp, NULL);
> drivers/scsi/gdth.c:    error = scsi_add_host(shp, NULL);
> drivers/scsi/gvp11.c:   error = scsi_add_host(instance, NULL);
> drivers/scsi/imm.c:     err = scsi_add_host(host, NULL);
> drivers/scsi/pcmcia/fdomain_stub.c:    if (scsi_add_host(host, NULL))
> drivers/scsi/pcmcia/nsp_cs.c:   ret = scsi_add_host (host, NULL);
> drivers/scsi/pcmcia/qlogic_stub.c:      if (scsi_add_host(shost, NULL))
> drivers/scsi/pcmcia/sym53c500_cs.c:     if (scsi_add_host(host, NULL))
> drivers/scsi/ppa.c:     err = scsi_add_host(host, NULL);
> drivers/scsi/qlogicfas.c:       if (scsi_add_host(hreg, NULL))
> drivers/scsi/scsi_module.c:             error = scsi_add_host(shost, NULL);
> drivers/scsi/sgiwd93.c: err = scsi_add_host(host, NULL);

Maybe the DMA API should be coping with NULL device? It seems to handle
it in a half of the methods and fails in the other half...

Pawel

Patch

diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
index 3cbb57a8b846..d8d3b294f5bc 100644
--- a/drivers/scsi/hosts.c
+++ b/drivers/scsi/hosts.c
@@ -218,16 +218,16 @@  int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 		goto fail;
 
 	if (!shost->shost_gendev.parent)
-		shost->shost_gendev.parent = dev ? dev : &platform_bus;
-	if (!dma_dev)
-		dma_dev = shost->shost_gendev.parent;
-
-	shost->dma_dev = dma_dev;
+		shost->shost_gendev.parent = dev;
 
 	error = device_add(&shost->shost_gendev);
 	if (error)
 		goto out;
 
+	if (!dma_dev)
+		dma_dev = shost->shost_gendev.parent;
+	shost->dma_dev = dma_dev;
+
 	pm_runtime_set_active(&shost->shost_gendev);
 	pm_runtime_enable(&shost->shost_gendev);
 	device_enable_async_suspend(&shost->shost_gendev);