linux-kernel - Re: [RFC PATCH 1/2] memremap: add arch specific hook for MEMREMAP

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Date:	Thu, 25 Feb 2016 08:49:47 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Nicolas Pitre <nico@...aro.org>
Subject: Re: [RFC PATCH 1/2] memremap: add arch specific hook for MEMREMAP_WB mappings

On 23 February 2016 at 23:23, Robert Jarzmik <robert.jarzmik@...e.fr> wrote:
> Dan Williams <dan.j.williams@...el.com> writes:
>
>> On Tue, Feb 23, 2016 at 4:26 AM, Ard Biesheuvel
>> <ard.biesheuvel@...aro.org> wrote:
>>> On 23 February 2016 at 13:03, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>>>> On 23 February 2016 at 12:58, Russell King - ARM Linux
>>>> <linux@....linux.org.uk> wrote:
>>>>> On Mon, Feb 22, 2016 at 09:35:24PM +0100, Ard Biesheuvel wrote:
>>> OK, I see what you mean. I find it unfortunate that ioremap_cache()
>>> instances are blindly being replaced with memremap(), and I wonder if
>>> this wasted test by and/or cc'ed to people who can actually test this
>>> driver. Dan?
>
> Actually I have the hardware to test it.
>
> And I also know what is behind :
>  - it's a CFI NOR based memory
>  - these are Intel StrataFlash 28F128J3A chips
>  - as a CFI memory it is mapped on the system bus
>  - from a read perspective, it behaves like a normal memory
>  - but once the first write reaches the CFI, everything changes (the address
>    space layout doesn't have the same meaning, be that becoming a status code or
>    something else).
>    In these conditions reordering of writes versus reads, merging reads after
>    a write or coalescing writes is a recipe for disaster.
>
> All of this to say I can make a small discrete number of tests (less than 10
> write or erase ones to preserve the precious NOR).
>

Thanks Robert.

But to be honest, I think we should simply revert the change, after
which we can wire up memremap() for ARM properly. And while I agree
that ioremap_cache() is often abused for mapping things like ACPI
tables in RAM (which forces you to cast away the __iomem annotation),
using ioremap_cache() to map NOR flash is totally different IMO, even
if it has memory semantics while in array mode.