linux-kernel - Re: [PATCH v2 1/4] RISC-V: Fix ioremap_cache() and ioremap

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Message-Id: <45de6e04-b19b-4ffe-878e-6ba8123f2aef@www.fastmail.com>
Date:   Fri, 23 Sep 2022 12:35:50 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Palmer Dabbelt" <palmer@...belt.com>,
        "Anup Patel" <apatel@...tanamicro.com>
Cc:     "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Atish Patra" <atishp@...shpatra.org>,
        Heiko Stübner <heiko@...ech.de>,
        anup@...infault.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, mchitale@...tanamicro.com
Subject: Re: [PATCH v2 1/4] RISC-V: Fix ioremap_cache() and ioremap_wc() for systems
 with Svpbmt

On Thu, Sep 22, 2022, at 6:35 PM, Palmer Dabbelt wrote:
> On Thu, 15 Sep 2022 19:24:55 PDT (-0700), apatel@...tanamicro.com wrote:
>>
>> On Tue, Aug 30, 2022 at 10:17 AM Anup Patel <apatel@...tanamicro.com> wrote:
>>>
>>> Currently, all flavors of ioremap_xyz() function maps to the generic
>>> ioremap() which means any ioremap_xyz() call will always map the
>>> target memory as IO using _PAGE_IOREMAP page attributes. This breaks
>>> ioremap_cache() and ioremap_wc() on systems with Svpbmt because memory
>>> remapped using ioremap_cache() and ioremap_wc() will use _PAGE_IOREMAP
>>> page attributes.
>>>
>>> To address above (just like other architectures), we implement RISC-V
>>> specific ioremap_cache() and ioremap_wc() which maps memory using page
>>> attributes as defined by the Svpbmt specification.
>>>
>>> Fixes: ff689fd21cb1 ("riscv: add RISC-V Svpbmt extension support")
>>> Co-developed-by: Mayuresh Chitale <mchitale@...tanamicro.com>
>>> Signed-off-by: Mayuresh Chitale <mchitale@...tanamicro.com>
>>> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
>>
>> This is a crucial RC fix. Can you please take this ?
>
> Sorry I missed this, I thought it was just part of the rest of this 
> patch set.  That said, I'm not actually sure this is a critical fix: 
> sure it's a performance problem, and if some driver is expecting 
> ioremap_cache() to go fast then possibly a pretty big one, but the only 
> Svpmbt hardware that exists is the D1 and that was just supported this 
> release so it's not a regression.  Maybe that's a bit pedantic, but all 
> this travel has kind of made things a mess and I'm trying to make sure 
> nothing goes off the rails.

I think generally speaking any use of ioremap_cache() in a driver
is a mistake. The few users that exist are usually from historic
x86 specific code and are hard to kill off.

     Arnd