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Fri, 6 May 2022 09:38:55 GMT Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 015EA42042; Fri, 6 May 2022 09:38:55 +0000 (GMT) Received: from d06av24.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 30EBB42041; Fri, 6 May 2022 09:38:53 +0000 (GMT) Received: from sig-9-145-46-59.uk.ibm.com (unknown [9.145.46.59]) by d06av24.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 6 May 2022 09:38:53 +0000 (GMT) Message-ID: <157602011a72061dd31f92bd699e8c1f9a81c988.camel@linux.ibm.com> Subject: Re: [RFC v2 01/39] Kconfig: introduce HAS_IOPORT option and select it as necessary From: Niklas Schnelle To: Bjorn Helgaas , Arnd Bergmann , John Garry Date: Fri, 06 May 2022 11:38:52 +0200 In-Reply-To: <20220505195342.GA509942@bhelgaas> References: <20220505195342.GA509942@bhelgaas> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5 (3.28.5-18.el8) X-TM-AS-GCONF: 00 X-Proofpoint-GUID: Izd-B44nIbHN-ZNsHTvoLMo3Drr04sYh X-Proofpoint-ORIG-GUID: VHp-W17L3sYL52T90zfXFVsNVoEuLE10 Content-Transfer-Encoding: 7bit X-Proofpoint-UnRewURL: 0 URL was un-rewritten MIME-Version: 1.0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.858,Hydra:6.0.486,FMLib:17.11.64.514 definitions=2022-05-06_03,2022-05-05_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 mlxlogscore=748 clxscore=1011 impostorscore=0 mlxscore=0 phishscore=0 suspectscore=0 bulkscore=0 spamscore=0 lowpriorityscore=0 priorityscore=1501 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2205060052 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Rich Felker , "open list:IA64 \(Itanium\) PLATFORM" , "open list:SUPERH" , Catalin Marinas , Dave Hansen , "open list:MIPS" , "James E.J. Bottomley" , "open list:SPARC + UltraSPARC \(sparc/sparc64\)" , "open list:RISC-V ARCHITECTURE" , Will Deacon , linux-arch , Yoshinori Sato , Helge Deller , "maintainer:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" , Russell King , Ingo Molnar , Geert Uytterhoeven , linux-pci , Matt Turner , Albert Ou , Arnd Bergmann , "open list:M68K ARCHITECTURE" , Ivan Kokshaysky , Paul Walmsley , Thomas Gleixner , "moderated list:ARM PORT" , Richard Henderson , Michal Simek , Thomas Bogendoerfer , "open list:PARISC ARCHITECTURE" , Greg Kroah-Hartman , Linux Kernel Mailing List , Palmer Dabbelt , "open list:ALPHA PORT" , Borislav Petkov , "open list:LINUX FOR POWERPC \(32-BIT AND 64-BIT\)" , "David S. Miller" Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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 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@huawei.com/