Re: [PATCH v2 1/4] RISC-V: Fix ioremap_cache() and ioremap_wc() for systems with Svpbmt

[Arnd Bergmann]

On Thu, Sep 22, 2022, at 6:35 PM, Palmer Dabbelt wrote:
> On Thu, 15 Sep 2022 19:24:55 PDT (-0700), apatel@xxxxxxxxxxxxxxxx wrote:
>>
>> On Tue, Aug 30, 2022 at 10:17 AM Anup Patel <apatel@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx>
>>> Signed-off-by: Mayuresh Chitale <mchitale@xxxxxxxxxxxxxxxx>
>>> Signed-off-by: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
>>
>> 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