Re: [PATCH v3 06/18] tegra: fdt: Add LCD definitions for Tegra

[Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>]

On 09/27/2012 02:27 PM, Simon Glass wrote:
> On Thu, Sep 27, 2012 at 8:49 AM, Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.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:
> 
> 1. 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.
> 
> 2. 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.