On 09/27/2012 02:27 PM, Simon Glass wrote:
On Thu, Sep 27, 2012 at 8:49 AM, Stephen Warren swarren@wwwdotorg.org wrote:
On 09/27/2012 07:58 AM, Simon Glass wrote:
...
Really this is just a way of getting U-Boot and Linux to agree on the address, by having U-Boot know the address that the kernel will happen to use. It isn't very robust, but we have found it useful as a means of avoiding problems in this area.
Right, but again, why can't the display driver simply read the address out of the register; why is there a need to duplicate the data?
I guess one possibility is that the register only gives the base address and not the size/mode of the allocated surface, but I don't recall if this proposed binding did that either?
So here is my explanation:
- U-Boot would normally put the display right near the top of memory.
Instead, it figures out where the kernel will put it (somehow this seems to often be at a fixed address) and uses that address.
- That means that U-Boot will now have the display exactly where the
kernel wants it.
Oh, the DT property is telling U-Boot where to put the surface so that when the kernel determines where to put it, it'll already be there?
That seems completely backwards. It's also extremely fragile; what if the kernel suddenly starts allocating at some other random address, which stops matching what U-Boot expects?
Far better would be for U-Boot to put the surface wherever it wants, then manipulate the DT that's passed to the kernel to:
a) Add a /memreserve/ so that memory doesn't get re-used for anything else during boot.
b) Add a property to the display node indicating which memreserve represents the frame-buffer location.
When the kernel boots, it can:
* Copy the content from the U-Boot-allocated buffer to whatever display buffer the kernel allocated before writing the new address to the HW.
* Undo the memory reservation triggered by the /memreserve/ to allow the memory to be re-used.
So what do you think? Should we remove this entirely, or is it a useful hack?
Yes, I think in its current form, it isn't useful.