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

On Wed, Feb 24, 2016 at 01:21:51PM +0200, Tomi Valkeinen wrote:

On 24/02/16 12:46, Russell King - ARM Linux wrote:

quoted

On Tue, Feb 23, 2016 at 02:45:30PM +0200, Tomi Valkeinen wrote:

quoted

My opinion is that the bootloader should be responsible for telling the
kernel what hardware there is on the board. For busses like PCI we have
proper probing mechanism with global unique identifiers for the devices,
and nothing is needed from the bootloader.

Exactly, but that is _NOT_ the case here, because we're not talking
about an on-board display.

Ok, what is it then? I'm not familiar with the boards in question.

When does a display become an on-board display? All the panels I have
can be disconnected quite easily, but I still consider them as on-board
displays.

The difference to me is quite clear.

If the connector is a flexi-strip or LVDS connector designed to be
connected directly to a panel, it is not designed as a user connector,
and the display can be regarded as part of the board: the connector
probably isn't rated for a large number of mating cycles.

If the connector is a board-edge external-unit connector, then the
panel is not part of the board.

In the case of Versatile, it's the latter: the connector is situated
at the board edge, next to the serial port connectors, and is designed
to connect to an external box housing the display.

quoted

That argument does not stack up.  Just because you've plugged in a
network device does not mean that the kernel can drive it: the kernel
needs a device specific driver, which is determined by looking at the
IDs.  There is no standard network driver PCI interface.

Yes, but you can connect the network device to any board with a PCI bus
and it works. Here, if I'm not mistaken, the displays are built for this
single board, making them board specific.

It only works because Linux has a rich array of network drivers supporting
all that hardware, and the appropriate network driver is bound depending
on the hardware ID of the card.  If a new PCI network device comes out,
it'll more likely than not require an update to a network driver to make
it work.

The displays are not built for "this single board" but for a family of
boards: not only Versatile PB/AB, but also the Realview family of boards
too.

But I cannot accept that as a general solution for all similar cases
going forward, especially when moving to DRM world, that's just bad SW
design.

I think that's a matter of personal opinion, perspective and situation.
What is good design today is not necessary good design yesterday or
tomorrow.  I thought we already ascertained that earlier in this
discussion. :)

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