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

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

On 11/10/20 10:36 AM, Ahmad Fatoum wrote:
> 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

Or rather a  more generic name, like dev_ioremap?.(In that case
!CONFIG_OF is the same as MMU off).

>   - 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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