Re: [PATCH] ARM: keystone: add bus notifier to set dma_pfn_offset for pci devices

[Santosh Shilimkar]

On 10/10/14 11:42 AM, Russell King - ARM Linux wrote:
> On Fri, Oct 10, 2014 at 11:29:03AM -0400, Santosh Shilimkar wrote:
> > On 10/10/14 11:15 AM, Murali Karicheri wrote:
> > > When PCI device driver such as that for e1000e tries to set dma mask
> > > using dma_set_mask_and_coherent(), it fails because the dma_pfn_offset
> > > is incorrect on a Keystone SoC. This patch fix this by adding a bus
> > > notifier to set this correctly for PCI devices.
> > >
> > > Signed-off-by: Murali Karicheri <m-karicheri2@xxxxxx>
> > > ---
> > Looks good. I will pick this up after the merge window.
>
> No it doesn't, this patch is crap.  Really.  Let's look again at what the
> patch is doing:
>
>          if (platform_nb.notifier_call)
>                  bus_register_notifier(&platform_bus_type, &platform_nb);
> +       if (platform_nb.notifier_call)
> +               bus_register_notifier(&pci_bus_type, &platform_nb);
>
> Notice that both calls are using the same platform_nb structure, which is:
>
> static struct notifier_block platform_nb;
>
> and in turn this is:
>
> struct notifier_block {
>          notifier_fn_t notifier_call;
>          struct notifier_block __rcu *next;
>          int priority;
> };
>
> Notice that "next" pointer - these blocks are used as a single-linked list.
> So, this block gets registered for the platform bus, and is inserted into
> that bus notifier chain.  That means "next" may be set to a non-NULL
> next notifier block.
>
> Then it gets registered against the PCI bus, which *will* overwrite the
> next pointer in platform_nb.
Err.... You are dead right. I missed completely that it is using the
same notifier block. Sorry for oversight and thanks for spotting it.

Regards,
Santosh

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