Re: [PATCH 11/11] ARM: versatile: move CLCD configuration to device tree

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Tue, Feb 23, 2016 at 11:56:34AM +0000, Peter Maydell wrote:
> On 23 February 2016 at 09:58, Tomi Valkeinen <tomi.valkeinen@ti.com> wrote:
> > I'm not quite sure how it works if, as in versatile display case, there
> > are multiple DT overlays of which one has to be enabled. I hope there's
> > support to choose which one to use via kernel cmdline or similar.
> >
> > I would personally like it much more if the bootloader would either
> > compose a final dtb from DT fragments and pass it to the kernel, or
> > alternatively the bootloader would pass the base dtb image and a bunch
> > of DT overlays to the kernel, and the kernel would deal with the DT
> > overlays.
> 
> Speaking as somebody who's written the "bootloader" code that's
> used for what I guess are the majority of versatile kernel boots,
> i.e. the one in QEMU, I think that requiring the bootloader to do this
> would be a significant worsening from the current state.
> 
> Right now the bootloader doesn't need to do much at all with device
> trees, except pass the kernel the DT that the user gave us, which
> is just the kernel's own data structures in a separate file for
> convenience. You need to do some very minor tweaks to the /chosen
> node, but these can be handled the same way for any board and aren't
> hardware specific. There's no need to worry about dt fragments
> either for the bootloader or for the user. Imposing a new requirement
> for the bootloader to have to probe hardware which it otherwise
> has no need to even care about, and then edit and update the DT
> in a board-specific manner, or have board-specific DT fragments,
> seems like a totally unnecessary imposition on both bootloader
> authors and end-users, and of course it would break booting newer
> kernels on the great mass of already existing boot loaders and
> QEMU installs.
> 
> The kernel is in a position to probe the display hardware and determine
> what is there, and do the right thing, and that's exactly what it
> does today. The kernel should continue to do this.
> The advantage of DT is that it allows moving information about
> non-probeable hardware that was previously hardwired in the kernel
> C sources into a separate data structure, but the versatile displays
> are not non-probeable. I can see no benefit at all from hardwiring into
> the dt something which the kernel has previously been successfully
> dynamically getting right without any bootloader intervention -- it just
> makes the kernel less flexible and less user-friendly.

+1.

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