On Thu, 2022-05-05 at 14:53 -0500, Bjorn Helgaas wrote: > On Thu, May 05, 2022 at 07:39:42PM +0200, Arnd Bergmann wrote: > > On Thu, May 5, 2022 at 6:10 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote: > > > On Wed, May 04, 2022 at 11:31:28PM +0200, Arnd Bergmann wrote: > > > > The main goal is to avoid c), which is what happens on s390, but > > > > can also happen elsewhere. Catching b) would be nice as well, > > > > but is much harder to do from generic code as you'd need an > > > > architecture specific inline asm statement to insert a ex_table > > > > fixup, or a runtime conditional on each access. > > > > > > Or s390 could implement its own inb(). > > > > > > I'm hearing that generic powerpc kernels have to run both on machines > > > that have I/O port space and those that don't. That makes me think > > > s390 could do something similar. > > > > No, this is actually the current situation, and it makes absolutely no > > sense. s390 has no way of implementing inb()/outb() because there > > are no instructions for it and it cannot tunnel them through a virtual > > address mapping like on most of the other architectures. (it has special > > instructions for accessing memory space, which is not the same as > > a pointer dereference here). > > > > The existing implementation gets flagged as a NULL pointer dereference > > by a compiler warning because it effectively is. > > I think s390 currently uses the inb() in asm-generic/io.h, i.e., > "__raw_readb(PCI_IOBASE + addr)". I understand that's a NULL pointer > dereference because the default PCI_IOBASE is 0. > > I mooted a s390 inb() implementation like "return ~0" because that's > what happens on most arches when there's no device to respond to the > inb(). > > The HAS_IOPORT dependencies are fairly ugly IMHO, and they clutter > drivers that use I/O ports in some cases but not others. But maybe > it's the most practical way. > > Bjorn I fear such stubs are kind of equivalent to my previous patch doing the same in asm-generic/io.h that was pulled and then unpulled by Linus. Maybe it would be slightly different if instead of a warning outX() would just be a NOP and inX() just returned ~0 but we're in essence pretending that we have these functions when we know they are nonsense. Another argument I see is that as shown by POWER9 we might start to see more platforms that just can't do I/O port access. E.g. I would also be surprised if Apple's M1 has I/O port access. Sooner or later I expect distributions on some platforms to only support such systems. For example on ppc a server distribution might only support IBM POWER without I/O port support before too long. Then having HAS_IOPORT allows to get rid of drivers that won't work anyway. There are also reports of probing a driver with I/O ports causing a system crash on systems without I/O port support. For example in this answer by John Garry (added so he may supply more information): https://lore.kernel.org/lkml/db043b76-880d-5fad-69cf-96abcd9cd34f@xxxxxxxxxx/
