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SubjectRe: avoiding pci_disable_device()...
Greg KH wrote:
> On Sun, Feb 13, 2005 at 08:42:55PM -0500, Jeff Garzik wrote:
>
>>Currently, in almost every PCI driver, if pci_request_regions() fails --
>>indicating another driver is using the hardware -- then
>>pci_disable_device() is called on the error path, disabling a device
>>that another driver is using
>>
>>To call this "rather rude" is an understatement :)
>>
>>Fortunately, the ugliness is mitigated in large part by the PCI layer
>>helping to make sure that no two drivers bind to the same PCI device.
>>Thus, in the vast majority of cases, pci_request_regions() -should- be
>>guaranteed to succeed.
>>
>>However, there are oddball cases like mixed PCI/ISA devices (hello IDE)
>>or cases where a driver refers a pci_dev other than the primary, where
>>pci_request_regions() and request_regions() still matter.
>
>
> But this is a very small subset of pci devices, correct?

No. You also need to consider situations such as out-of-tree drivers
for the same hardware (might not use PCI API), and situations where you
have peer devices discovered and used (PCI API doesn't have "hey, <this>
device is associated with <current driver>, too" capability)

>>As a result, I have committed the attached patch to libata-2.6. In many
>>cases, it is a "semantic fix", addressing the case
>>
>> * pci_request_regions() indicates hardware is in use
>> * we rudely disable the in-use hardware
>>
>>that would not occur in practice.
>>
>>But better safe than sorry. Code cuts cut-n-pasted all over the place.
>>
>>I'm hoping one or two things will happen now:
>>* janitors fix up the other PCI drivers along these lines
>>* improve the PCI API so that pci_request_regions() is axiomatic
>
>
> Do you have any suggestions for how to do this?

I'm glad you asked ;-) As the author of pci_disable_device() and
pci_request_regions(), I recognized their inadequacy almost immediately.

There are some fundamental flaws in the API that need correcting:

* pci_disable_device() should perform exactly the opposite of
pci_enable_device(), no more, no less. It should NOT unconditionally
disable the device, but instead restore the hardware to the state it was
in prior to pci_enable_device().

* pci_request_regions() should be axiomatic. By that I mean,
pci_enable_device() should
(a) handle pci_request_regions() completely
(b) fail if regions are not available

* pci_enable_device() may touch the hardware when it should not. In an
ideal world, pci_enable_device() would
* assign resources to device, if necessary
* request_resource()s [aka pci_request_regions()]
* enable device by setting bits in PCI_COMMAND, etc.
but since the request-resource step is assumed to occur after
pci_enable_device() returns to the driver, this is impossible.


The solution? I am still thinking. My gut feeling is that we want a
slightly higher level PCI API for drivers. Drivers pass in an 'info'
structure to pci_up(). pci_up() enables the device, requests resources
(not just irq), maps resources as necessary, enables irqs and/or MSI as
necessary, and similar housekeeping. pci_down() does the precise
opposite. Essentially, pci_up() is a lib function that kills a ton of
duplicate code from the vast majority of PCI drivers.


OTOH, Alan's suggestion seems sane and a lot more simple, but doesn't
address the flaws in the API.

Jeff


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