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

From: Santosh Shilimkar
Date: Fri Oct 10 2014 - 11:47:28 EST




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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