Re: powerpc/powernv/pci-ioda: fix kdump with non-power-of-2 crashkernel=

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 04.09.2015 [20:01:22 +0200], Jan Stancek wrote:
> On Fri, Sep 04, 2015 at 09:59:38AM -0700, Nishanth Aravamudan wrote:
> > The 32-bit TCE table initialization relies on the DMA window having a
> > size equal to a power of 2 (and checks for it explicitly). But
> > crashkernel= has no constraint that requires a power-of-2 be specified.
> > This causes the kdump kernel to fail to boot as none of the PCI devices
> > (including the disk controller) are successfully initialized.
> > 
> > After this change, the PCI devices successfully set up the 32-bit TCE
> > table and kdump succeeds.
> > 
> > Fixes: aca6913f5551 ("powerpc/powernv/ioda2: Introduce helpers to allocate TCE pages")
> > Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
> > Cc: stable@vger.kernel.org # 4.2
> > ---
> > 
> > Michael, I did this as a follow-on patch to my previous one. If you'd
> > rather I made a v3 of that patch with the two fixes combined, I can
> > resend.
> > 
> > diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
> > index f1c74c28e564..73914f4bd1ab 100644
> > --- a/arch/powerpc/platforms/powernv/pci-ioda.c
> > +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
> > @@ -2084,6 +2084,12 @@ static long pnv_pci_ioda2_setup_default_config(struct pnv_ioda_pe *pe)
> >  	 */
> >  	const u64 window_size =
> >  		min((u64)pe->table_group.tce32_size, memory_hotplug_max());
> > +	/*
> > +	 * crashkernel= specifies the kdump kernel's maximum memory at
> > +	 * some offset and there is no guaranteed the result is a power
> > +	 * of 2, which will cause errors later.
> > +	 */
> > +	window_size = __rounddown_pow_of_two(window_size);
> 
> Hi,
> 
> Just wondering if this won't hide potential alignment issues of
> "table_group.tce32_size", that now trigger EINVAL down the road
> and if it wouldn't be safer to round only "memory_hotplug_max()?
> 
>   const __u64 hotplug_max_p2 = __rounddown_pow_of_two(memory_hotplug_max());
>   const __u64 window_size =
>             min((u64)pe->table_group.tce32_size, hotplug_max_p2);

Fair point, v2 on its way.