Messages in this thread | | | Date | Fri, 10 Oct 2014 11:53:44 -0400 | From | Murali Karicheri <> | Subject | Re: [PATCH] ARM: keystone: add bus notifier to set dma_pfn_offset for pci devices |
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On 10/10/2014 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@ti.com> >>> --- >> 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. > > There are several side effects from this: > > 1. Any subsequent notifiers on the platform bus which come after _this_ > notifier are now orphaned, and will never be called. > > 2. Any subsequent notifiers on the PCI bus list which come after _this_ > notifier will now also be called for the platform bus. > > 3. Subsequent notifiers registered against either list which are sorted > after _this_ notifier will be attached to _both_ lists. > > In other words, this patch totally screws up the notifier lists for both > buses, and while it may not be immediately obvious, if any of the above > three scenarios occur, it will probably be very confusing to debug. > > So, one hell of a big NAK on this patch. > > Moreover, I have to ask why there wasn't some research done first into > how notifiers work *before* writing this code, specifically to find out > whether it is safe to register the same notifier block simultaneously > onto two lists. > Thanks Russel for the comments. I didn't see any issues when I tested my PCI driver with this patch, but as you have pointed out there are side effects. I will work to address your concerns.
Murali out,
-- Murali Karicheri Linux Kernel, Texas Instruments
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