On Wed, Apr 25, 2018 at 01:54:39AM -0700, Christoph Hellwig wrote: > [discussion about this patch, which should have been cced to the iommu > and linux-arm-kernel lists, but wasn't: > https://www.spinics.net/lists/dri-devel/msg173630.html] > > On Wed, Apr 25, 2018 at 09:41:51AM +0200, Thierry Reding wrote: > > > API from the iommu/dma-mapping code. Drivers have no business poking > > > into these details. > > > > The interfaces that the above patch uses are all EXPORT_SYMBOL_GPL, > > which is rather misleading if they are not meant to be used by drivers > > directly.
EXPORT_SYMBOL* means nothing as far as whether a driver should be able to use the symbol or not - it merely means that the symbol is made available to a module. Modules cover much more than just device drivers - we have library modules, filesystem modules, helper modules to name a few non-driver classes of modules. We also have symbols that are exported as part of the architecture implementation detail of a public interface. For example, the public interface "copy_from_user" is implemented as an inline function (actually several layers of inline functions) eventually calling into arm_copy_from_user(). arm_copy_from_user() is exported, but drivers (in fact no module) is allowed to make direct reference to arm_copy_from_user() - it'd fail when software PAN is enabled. The whole idea that "if a symbol is exported, it's fine for a driver to use it" is a complete work of fiction, always has been, and always will be. We've had this with the architecture implementation details of the DMA API before, and with the architecture implementation details of the CPU cache flushing. There's only so much commentry, or __ prefixes you can add to a symbol before things get rediculous, and none of it stops people creating this abuse. The only thing that seems to prevent it is to make life hard for folk wanting to use the symbols (eg, hiding the symbol prototype in a private header, etc.) Never, ever go under the covers of an interface. If the interface doesn't do what you want, _discuss_ it, don't just think "oh, that architecture private facility looks like what I need, I'll use that directly." If you ever are on the side of trying to maintain those implementation details that are abused in this way, you'll soon understand why this behaviour by driver authors is soo annoying, and the maintainability problems it creates. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 8.8Mbps down 630kbps up According to speedtest.net: 8.21Mbps down 510kbps up
