Re: [PATCH 2/8] mtd: cfi_flash: allow 0x0 mapping

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hi,

On 11/10/20 10:15 AM, Sascha Hauer wrote:
> On Tue, Nov 10, 2020 at 08:48:11AM +0100, Sascha Hauer wrote:
>> On Tue, Nov 10, 2020 at 07:33:53AM +0100, Rouven Czerwinski wrote:
>>> On Mon, 2020-11-09 at 14:52 +0100, Lucas Stach wrote:
>>>> Am Montag, den 09.11.2020, 14:44 +0100 schrieb Rouven Czerwinski:
>>>>> Annotate the different read and write functions with
>>>>> zero_page_{access/faulting}. This allows the cfi_flash driver to be used
>>>>> on the QEMU virt machine with an enabled MMU.
>>>>
>>>> I don't like this zero-page access allow in a driver at all. If you
>>>> have some free address space somewhere, you could also solve this issue
>>>> by remapping the IO resource to somewhere else in the address space,
>>>> deviating from the 1:1 mapping. The Tegra PCIe host driver does this,
>>>> if you need some inspiration.
>>>
>>> I totally agree, remapping the IO region sounds like a much better
>>> choice.
>>
>> But where should it be mapped to? Just 4k upwards and hope that the area
>> just above the cfi flash isn't occupied by something else?
> 
> Ok, here's the plan:
> 
> In a board specific initcall map the memory to wherever it's convenient,
> modify the address of the CFI flash in the device node and add a big
> comment that there's nothing to see here.

The device tree specification defines a "virtual-reg property [that] specifies
an effective address that maps to the first physical address specified in
the reg property of the device node.  This property enables boot programs to
provide client programs with virtual-to-physical mappings that have been set up".

So, how about a tad more generic approach:

- barebox device tree extends cfi-flash node with virtual-reg of appropriate
  location
- Define a of_iomap() function that
  - MMU off => does normal request_iomem_region when MMU is off
  - MMU on  => remaps reg to virtual-reg and does request_iomem_region

Advantages:
  - No extra board code
  - Reusable for others as well
  - No device tree patching necessary

I am not familiar with the PowerPC support in barebox, but if we keep the MMU
on, then the use of virtual-reg is correct and if we turn it off, it doesn't
hurt to have it, so we should be within spec.

Let me know what you think.
Cheers,
Ahmad

> 
> Sascha
> 
> 

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