[Qemu-devel] Re: RFC: emulation of system flash

[Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>]

Auf 11.03.2011 01:19, Jan Kiszka schrieb:
> On 2011-03-10 23:10, Carl-Daniel Hailfinger wrote:
>   
>> Auf 10.03.2011 22:55, Jordan Justen schrieb:
>>     
>>> On Thu, Mar 10, 2011 at 13:37, Carl-Daniel Hailfinger
>>> <c-d.hailfinger.devel.2006@gmx.net> wrote:
>>>   
>>>       
>>>> Auf 10.03.2011 05:51, Jordan Justen schrieb:
>>>>     
>>>>         
>>>>> I have documented a simple flash-like device which I think could be
>>>>> useful for qemu/kvm in some cases.  (Particularly for allowing
>>>>> persistent UEFI non-volatile variables.)
>>>>>
>>>>> http://wiki.qemu.org/Features/System_Flash
>>>>>
>>>>> Let me know if you have any suggestions or concerns.
>>>>>
>>>>>       
>>>>>           
>>>> Is there any reason why you chose to invent an interface for the flash
>>>> chip which is more complicated than the interface used by common flash
>>>> chips out there?
>>>> Maybe some EFI requirement?
>>>>     
>>>>         
>>> This is a simpler version than the flash devices I'm used to dealing
>>> with for x86/x86-64 UEFI systems.  Can you suggest an alternative real
>>> interface that is simpler?
>>>   
>>>       
>> Pseudocode for the real interface most common on x86 before SPI came along:
>>
>> Write a page (256 bytes) or a few bytes:
>> chip_writeb(0xAA, bios_base + 0x5555);
>> chip_writeb(0x55, bios_base + 0x2AAA);
>> chip_writeb(0xA0, bios_base + 0x5555);
>> chip_writeb(databyte0, bios_base + addr);
>> chip_writeb(databyte1, bios_base + addr + 1);
>> chip_writeb(databyte2, bios_base + addr + 2);
>> chip_writeb(databyte3, bios_base + addr + 3);
>> ... up to 256 databytes.
>> chip_readb(bios_base);
>> The last chip_readb(bios_base) is repeated until the result does not
>> change anymore.
>>     
> Hmm, I may oversee some subtle difference, but this looks pretty much
> like CFI to me (see hw/pflash_cfi02.c).
>   

I thought CFI also implements a way to retrieve device size/geometry
with the Query (0x98) command, but that part is rarely implemented on
x86 flash (I didn't see any yet).
That said, the non-query commands are identical AFAICS.


> At least it's an in-band interface, which is the better choice as we
> currently only have a PIIX3 southbridge for x86, predating even FWHs.
>   

Right, that pretty much kills the option of using SPI unless someone
wants to emulate a flash translation controller (e.g. the ITE IT8716F
Super I/O). Can be done, would work, but the IT8716F has some quirks
(max 1 MB SPI flash chips) which make it a less desirable option for
emulation.


Regards,
Carl-Daniel

-- 
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