Re: [PATCH 0/3] mps3-an524: support memory remapping

[Philippe Mathieu-Daudé]

On 4/13/21 6:29 PM, Philippe Mathieu-Daudé wrote:> On 4/12/21 4:48 PM,
Peter Maydell wrote:
>> On Mon, 12 Apr 2021 at 15:37, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>>> On 4/12/21 3:43 PM, Peter Maydell wrote:
>>>> The AN524 FPGA image supports two memory maps, which differ
>>>> in where the QSPI and BRAM are. In the default map, the BRAM
>>>> is at 0x0000_0000, and the QSPI at 0x2800_0000. In the second
>>>> map, they are the other way around.
>>>>
>>>> In hardware, the initial mapping can be selected by the user
>>>> by writing either "REMAP: BRAM" (the default) or "REMAP: QSPI"
>>>> in the board configuration file. The guest can also dynamically
>>>> change the mapping via the SCC CFG_REG0 register.
>>>>
>>>> This patchset adds support for the feature to QEMU's model;
>>>> the user-sets-the-initial-mapping part is a new machine property
>>>> which can be set with "-M remap=QSPI".
>>>>
>>>> This is needed for some guest images -- for instance the
>>>> Arm TF-M binaries -- which assume they have the QSPI layout.
>>>
>>> I tend to see machine property set on the command line similar
>>> to hardware wire jumpers, externally set (by an operator having
>>> access to the hardware, not guest code).
>>>
>>> Here the remap behavior you described is triggered by the guest.
>>> Usually this is done by a bootloader code before running the
>>> guest code.
>>> Couldn't we have the same result using a booloader (like -bios
>>> cmd line option) rather than modifying internal peripheral state?
>>
> 
> (
> 
>> In the real hardware, the handling of the board configuration
>> file is done by the "Motherboard Configuration Controller", which
>> is an entirely separate microcontroller on the dev board but outside
>> the FPGA, and which is responsible for things like loading image
>> files off the SD card and writing them to memory, setting a bunch
>> of initial configuration including the remap setting but also
>> things like setting the oscillators to the values that this
>> particular FPGA image needs. It's also what makes the board
>> appear to a connected computer as a USB mass storage device so
>> you can update the SD card files via USB cable rather than doing
>> lots of plugging and unplugging, and it is what loads the FPGA
>> image off SD card and into the FPGA in the first place.
> 
> ) [*]
> 
>> QEMU is never going to implement the MCC as a real emulated
>> guest CPU; instead our models hard-code some of the things it
>> does. I think that a machine property (a thing set externally
>> to the guest CPU and valid before any guest CPU code executes)
>> is a reasonable way to implement the remap setting, which from
>> the point of view of the CPU inside the FPGA is a thing set
>> externally and valid before any guest CPU code executes.
> 
> OK now I understand the picture, the MCC is external. In that case
> the machine property is a clean way to address that.
> 
> Could you add the first paragraph of your answer ([*]) in patch 3
> description (before the current comment) to make it clearer?

(In case you agree, no need to respin).