linux-kernel - Re: [PATCH v2 02/22] asm-generic/io.h: add ioremap

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Date:   Thu, 6 Apr 2017 14:25:10 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Richard Henderson <rth@...ddle.net>,
        Tony Luck <tony.luck@...el.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Haavard Skinnemoen <hskinnemoen@...il.com>,
        Pratyush Anand <pratyush.anand@...il.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Mingkai Hu <mingkai.hu@...escale.com>,
        John Garry <john.garry@...wei.com>,
        Tanmay Inamdar <tinamdar@....com>,
        Murali Karicheri <m-karicheri2@...com>,
        Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>,
        Ray Jui <rjui@...adcom.com>,
        Wenrui Li <wenrui.li@...k-chips.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Minghuan Lian <minghuan.Lian@...escale.com>,
        Jon Mason <jonmason@...adcom.com>,
        Gabriele Paoloni <gabriele.paoloni@...wei.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
        Joao Pinto <Joao.Pinto@...opsys.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Michal Simek <michal.simek@...inx.com>,
        Stanimir Varbanov <svarbanov@...sol.com>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Roy Zang <tie-fei.zang@...escale.com>
Subject: Re: [PATCH v2 02/22] asm-generic/io.h: add ioremap_nopost remap
 interface

On Thu, Apr 06, 2017 at 01:11:57PM +0100, Russell King - ARM Linux wrote:
> On Thu, Apr 06, 2017 at 12:53:12PM +0200, Luis R. Rodriguez wrote:
> > On Thu, Apr 06, 2017 at 11:26:36AM +0100, Lorenzo Pieralisi wrote:
> > > Indeed, the static inline ioremap_nocache() fallback does not work
> > > on all arches (whether I add the fallback in linux/io.h or
> > > asm-generic/io.h is irrelevant), I bump into issues such as the one
> > > reported above.
> > 
> > Its also not *safe* to assume on behalf of all architectures a new ioremap
> > call is both a good idea and proper.
> 
> You may be right in general, but not in this case.
> 
> Currently, many drivers use plain ioremap() to map this resource.  We
> are replacing that existing call - which is known to work in the majority
> of cases - with a new call to cater for different semantics required by
> an architecture.
> 
> Doing a replace of these ioremap() calls with ioremap_nopost() in this
> situation, and then having ioremap_nopost() fail is a recipe for causing
> lots and lots of regressions.
> 
> The only sane approach is to have ioremap_post() default to modelling the
> _existing_ behaviour everywhere that it is used.
> 
> Requiring it to fail until architecture folk trip over the failure is
> totally insane, and I very strongly disagree with you on this.

Ah yes, what if with this modulo rule of thumb:

The litmus test then is if an existing set of calls are changed to
use a new ioremap then all archs that support those drivers where the new
call is being added must be modified to also have a correct corresponding
API call ?

This is more work on the new person introducing the new API, and should require
review still on arch maintainers but it seems like a fair compromise.

Then if an API is *new* though then things can move forward without requiring
all archs to add the respective call.

  Luis