From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bjorn Helgaas Date: Thu, 05 May 2022 16:10:28 +0000 Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary Message-Id: <20220505161028.GA492600@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 Wed, May 04, 2022 at 11:31:28PM +0200, Arnd Bergmann wrote: > On Wed, May 4, 2022 at 11:08 PM Bjorn Helgaas wrote: > > On Fri, Apr 29, 2022 at 03:49:59PM +0200, Niklas Schnelle wrote: > > > We introduce a new HAS_IOPORT Kconfig option to indicate support for > > > I/O Port access. In a future patch HAS_IOPORT=n will disable compilation > > > of the I/O accessor functions inb()/outb() and friends on architectures > > > which can not meaningfully support legacy I/O spaces such as s390 or > > > where such support is optional. > > > > So you plan to drop inb()/outb() on architectures where I/O port space > > is optional? So even platforms that have I/O port space may not be > > able to use it? > > > > This feels like a lot of work where the main benefit is to keep > > Kconfig from offering drivers that aren't of interest on s390. > > > > Granted, there may be issues where inb()/outb() does the wrong thing > > such as dereferencing null pointers when I/O port space isn't > > implemented. I think that's a defect in inb()/outb() and could be > > fixed there. > > The current implementation in asm-generic/io.h implements inb()/outb() > using readb()/writeb() with a fixed architecture specific offset. > > There are three possible things that can happen here: > > a) there is a host bridge driver that maps its I/O ports to this window, > and everything works > b) the address range is reserved and accessible but no host bridge > driver has mapped its registers there, so an access causes a > page fault > c) the architecture does not define an offset, and accessing low I/O > ports ends up as a NULL pointer dereference > > 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. Bjorn