On Thursday, June 2, 2016 9:00:01 AM CEST Bjorn Helgaas wrote: > On Wed, Jun 01, 2016 at 10:37:28PM +0200, Arnd Bergmann wrote: > > On Wednesday, June 1, 2016 2:04:30 PM CEST Bjorn Helgaas wrote: > > > On Wed, Jun 01, 2016 at 05:41:53PM +0200, Arnd Bergmann wrote: > > > > On Wednesday, June 1, 2016 10:09:29 AM CEST Bjorn Helgaas wrote: > > > > > Hi Arnd, > > > > > > > > > > On Wed, Jun 01, 2016 at 02:31:22PM +0200, Arnd Bergmann wrote: > > > > > > A lot of PCI host bridges require different methods for initiating > > > > > > type 0 and type 1 config space accesses, leading to duplication of > > > > > > code. > > > > > > > > > > > > This adds support for the two different kinds at the pci_ops > > > > > > level, with the newly added map_bridge/read_bridge/write_bridge > > > > > > operations for type 1 accesses. > > > > > > > > > > > > When these are not set, we fall back to the regular map_bus/read/write > > > > > > operations, so all existing drivers keep working, and bridges that > > > > > > have identical operations continue to only require one set. > > > > > > > > > > This adds new config accessor functions to struct pci_ops and makes > > > > > the callers responsible for figuring out which one to use. The > > > > > benefit is to reduce code duplication in some host bridge drivers > > > > > (DesignWare and MVEBU so far). > > > > > > > > > > From a design perspective, I'm not comfortable with moving this burden > > > > > from the host bridge drivers to the callers of the config accessors. > > > > ... > > > > > > > Maybe we can simply change them to use the normal API and come up with > > > > a way to make the pci_ops harder to misuse? Would it make you feel better > > > > if we also renamed .read/.write into .read_type0/.write_type0 or something > > > > like that? > > > > > > I'm trying to get a better feel for the tradeoff here. It seems like > > > an API complication vs. code duplication. > > > > > > I don't really think the callers should have to figure out which > > > accessor to use. How much of a benefit do we really gain by > > > complicating the callers? We've managed for quite a few years with > > > the current scheme, and it seems like only a couple new ARM platforms > > > would benefit. > > > > I just did a count of the implementations of pci_ops: I found 107 > > instances of 'struct pci_ops', and 67 of them treat type0 and type1 > > access differently in some form. > > > > I'd estimate that about half of them, or roughly a third of the total > > instances would benefit from my change, if we were to do them again. > > Clearly there is no need to change the existing code here when it works, > > unless the benefit is very clear and the code is actively maintained. > > > > In some cases, the difference is only that the root bus has a limited > > set of devices that are allowed to be accessed, so there would > > likely be no benefit of this, compared to e.g. yet another callback > > that checks the validity. > > Some other instances have type0 registers at a different memory location > > from type1, some use different layout inside of that space, and some > > are completely different. > > The type0/type1 distinction still seems out of place to me at the call > site. Is there any other reason a caller would care about the > difference between type0 and type1? Another idea based on my RFC patches to make pci_host_bridge the primary structure for probing PCI: we could split out the old 'bus::pci_ops' with the traditional read/write interface from a new structure that becomes pci_host_bridge::pci_host_bridge_ops, and also contains the other callbacks that we recently added to pci_ops, alongside type0/type1 accessors. We could then have a set of default pci_ops that call pci_host_bridge_ops->type0_read/type0_write/type1_read/type1_write, and those in turn get a pci_host_bridge as an argument along with the bus, device, function and register numbers instead of bus pointer and devfn/where. This way all existing code can keep working, but we can convert host drivers (if desired) to provide only pci_host_bridge_ops and no pci_ops, while making it easier to define those with a more modern interface. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
