From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bjorn Helgaas Date: Thu, 05 May 2022 19:53:42 +0000 Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary Message-Id: <20220505195342.GA509942@bhelgaas> List-Id: In-Reply-To: References: <20220429135108.2781579-2-schnelle@linux.ibm.com> In-Reply-To: <20220429135108.2781579-2-schnelle@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Arnd Bergmann Cc: Niklas Schnelle , Arnd Bergmann , Greg Kroah-Hartman , Linux Kernel Mailing List , linux-arch , linux-pci , Richard Henderson , Ivan Kokshaysky , Matt Turner , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Michal Simek , Thomas Bogendoerfer , "James E.J. Bottomley" , Helge Deller , Michael Ellerman , Paul Walmsley , Palmer Dabbelt , Albert Ou , Yoshinori Sato , Rich Felker , "David S. Miller" , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "open list:ALPHA PORT" , "moderated list:ARM PORT" , "open list:IA64 (Itanium) PLATFORM" , "open list:M68K ARCHITECTURE" , "open list:MIPS" , "open list:PARISC ARCHITECTURE" , "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" , "open list:RISC-V ARCHITECTURE" , "open list:SUPERH" , "open list:SPARC + UltraSPARC (sparc/sparc64)" On Thu, May 05, 2022 at 07:39:42PM +0200, Arnd Bergmann wrote: > On Thu, May 5, 2022 at 6:10 PM Bjorn Helgaas 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