Re: [Qemu-devel] [PATCHv2 1/2] sun4m: Add Sun CG3 framebuffer and corresponding OpenBIOS FCode ROM

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

On 09/02/14 15:33, Peter Maydell wrote:

>> It's been a little while since I looked, however this was my interpretation
>> of the table 3-12 on p.66 of the SBus specification. While that particular
>> table refers to the acknowledgment cycle from the slave back to the master,
>> it seems to work here in the same way when transferring between the master
>> and the slave.
>
> It's not that I think your comment is wrong about how SBus does
> byte transfers, or that the code is wrong, I just don't think that the
> reason for the code is the SBus behaviour; it's the device behaviour.
> QEMU abstracts a bus's mechanisms for transferring data into the
> MemoryRegion ops, which means that for a byte access you'll
> always get called with the byte value in the low 8 bits of the input
> argument. This would be true whether SBus passed byte values in
> lines 31..28, 7..0 or by sending them one bit at a time on D13. The
> relation between a byte write and a word write here is a property
> of the device, not the bus.
>
> (You'll notice that 3-12 allows the slave to send the byte data
> in various different lanes depending on which size acknowlegement
> it chooses to send. The documentation of what the master does
> is on page 53 and actually says you have to send it in two lanes
> for a non-aligned address, and certainly QEMU won't do anything
> corresponding to that. In any case I don't think it's relevant here.)

(goes and thinks about this for a bit)

Okay I can agree with that it's more an artifact of the implementation, 
rather than dependent upon the behaviour of the bus. I'll alter the 
comment for the next revision which I hope to post to the list over the 
next day or so.


ATB,

Mark.