linux-kernel - Re: [PATCH v4 2/4] RISC-V: Fix ioremap_cache() and ioremap

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Message-Id: <20bfd8a3-3836-46b8-9fdd-a407973224ba@app.fastmail.com>
Date:   Wed, 19 Oct 2022 22:50:17 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Anup Patel" <apatel@...tanamicro.com>,
        Heiko Stübner <heiko@...ech.de>
Cc:     "Palmer Dabbelt" <palmer@...belt.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Atish Patra" <atishp@...shpatra.org>,
        "Anup Patel" <anup@...infault.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        "Mayuresh Chitale" <mchitale@...tanamicro.com>
Subject: Re: [PATCH v4 2/4] RISC-V: Fix ioremap_cache() and ioremap_wc() for systems
 with Svpbmt

On Wed, Oct 19, 2022, at 18:10, Anup Patel wrote:
> On Wed, Oct 19, 2022 at 7:49 PM Heiko Stuebner <heiko@...ech.de> wrote:
>>
>> Am Mittwoch, 19. Oktober 2022, 15:11:26 CEST schrieb Anup Patel:
>> > 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>
>>
>> Wasn't there discussion around those functions in general in v2?
>
> Yes, there was discussion about a few drivers using ioremap_xyz()
> which is discouraged and drivers should use memremap().
>
> We still need the arch specific ioremap_xyz() functions/macros
> added by this patch because these are required by the generic
> kernel memremap() implementation (refer, kernel/iomem.c).

There is a difference between the strongly discouraged
ioremap_cache() that pretty much has no valid users, and
the ioremap_wt/ioremap_wc functions that are sometimes used
for mapping video framebuffer or similar.

It should be sufficient to provide a arch_memremap_wb()
and no ioremap_cache() to make memremap() work correctly.

      Arnd