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

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Date:   Fri, 23 Sep 2022 03:45:49 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     apatel@...tanamicro.com, Paul Walmsley <paul.walmsley@...ive.com>,
        atishp@...shpatra.org, 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 Fri, 23 Sep 2022 03:35:50 PDT (-0700), Arnd Bergmann wrote:
> 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.

Should we just add some sort of CONFIG_ARCH_HAS_IOREMAP_CACHE and then 
ban those drivers from everywhere else?