[U-Boot] [PATCH 3/5] arm: tegra30: video: integrate display driver for t30

[Simon Glass]

+Nikita

Hi Thierry,

On 24 August 2015 at 04:12, Thierry Reding <treding at nvidia.com> wrote:
> On Fri, Aug 21, 2015 at 06:37:37PM -0600, Simon Glass wrote:
> [...]
>> I have serious doubts about the wisdom of requiring a contributor to
>> completely re-architect the existing display system in U-Boot. It's a
>> big job. Perhaps we can settle for following along the same lines and
>> not making things worse?
>
> I didn't suggest re-architecting the display system in U-Boot. What I
> was suggesting was a way to architect Tegra-specific display driver code
> to make it reusable rather than duplicate display controller programming
> for each new generation, while the hardware has remained mostly the
> same.

OK, I misunderstood.

>
>> > Perhaps something as simple as:
>> >
>> >         struct tegra_dc {
>> >                 ...
>> >                 int (*enable)(struct tegra_dc *dc, const struct display_mode *mode);
>> >                 void (*disable)(struct tegra_dc *dc);
>> >                 ...
>> >         };
>> >
>> >         struct tegra_output {
>> >                 ...
>> >                 struct tegra_dc *dc;
>> >                 ...
>> >                 int (*enable)(struct tegra_output *output, const struct display_mode *mode);
>> >                 void (*disable)(struct tegra_output *output);
>> >                 ...
>> >         };
>> >
>> > would work fine. That's roughly how drivers are implemented in the
>> > kernel. Setting up display on an output would be done by determining the
>> > mode (typically by parsing EDID if available, or using a hard-coded mode
>> > otherwise) and then calling:
>> >
>> >         output->dc = dc;
>> >         dc->enable(dc, mode);
>> >         output->enable(output, mode);
>> >
>> > You might want to add in an abstraction for panels as well to make sure
>> > you have enough flexibility to enable and disable those, too. In that
>> > case you'd probably want to complement the above sequence with:
>> >
>> >         panel->enable(panel);
>>
>> Please don't add function points to structures on an ad-hoc basis.
>> These should use driver model. There is a uclass for display port but
>> not for LCD panels or SOR. You could add a very simple one for a panel
>> if you like. Please take a look at tegra124's display driver for an
>> example.
>
> I don't think the driver model is a good fit here. Abstracting a display
> port isn't very useful in itself because users don't really care about
> the type of display, they only care about it being a display. So if you
> want to usefully abstract you'd do it at a higher level, such as display
> or screen. Then you have a generic object which users can use to put up
> a framebuffer onto a physical screen.

I think you are referring to the lcd/video interface. If so, this is
already fairly well defined, but lcd and video should be merged, and a
uclass could be added. Nikita Kiryanov has done quite a bit of work on
the merging side.

But I still think there is value in a low-level abstraction too.
Function pointers indicate that there is an interface that can be used
by multiple drivers, and that is what driver model is for. See
displayport.h for an attempt at this. We can of course consider
expanding the display port uclass to encompass panels in general. I
was reluctant to do that with a sample size of one. Here is the
current interface:

/**
 * display_port_read_edid() - Read information from EDID
 *
 * @dev:        Device to read from
 * @buf:        Buffer to read into (should be EDID_SIZE bytes)
 * @buf_size:   Buffer size (should be EDID_SIZE)
 * @return number of bytes read, <=0 for error
 */
int display_port_read_edid(struct udevice *dev, u8 *buf, int buf_size);

/**
 * display_port_enable() - Enable a display port device
 *
 * @dev:        Device to enable
 * @panel_bpp:  Number of bits per pixel for panel
 * @timing:     Display timings
 * @return 0 if OK, -ve on error
 */
int display_port_enable(struct udevice *dev, int panel_bpp,
                        const struct display_timing *timing);

>
> SOR is an even worse abstraction because it's completely Tegra-specific
> and other SoCs will have completely different ways of providing the same
> types of output. You'll end up with a uclass containing a single
> implementation.

But if it is a single implementation why do you need to add function
pointers? It would just be a normal call in that case. I'm not
suggesting we add uclasses with no generic use.

>
> So, to reiterate, the above wasn't meant to be a generic abstraction for
> a U-Boot-wide display framework, but rather a suggestion on how the
> Tegra driver could internally be structured in order to avoid code
> duplication.
>
>> > Which should work for everything, except maybe DSI, where you may need
>> > some sort of inbetween step for panels that need additional setup using
>> > DCS commands or the like. But I suspect that's a bridge that can be
>> > crossed when we get to it.
>> >
>> > That said, I don't forsee myself having any time to devote to this, but
>> > if anyone ends up spending work on this, feel free to Cc me on patches
>> > or ask if you have questions about the display hardware or the framework
>> > design. I'm sure I can find the time to provide feedback.
>>
>> In which case I suggest we limit the amount of rewrite we ask for in
>> this case...
>
> People asked for my opinion, so I shared. If you prefer code duplication
> over a properly architected driver that's of course your prerogative.

I am wondering if the problem here is just that I misunderstood your
intent. How about:

- the display controller code (display.c) should be common across all Tegra SoCs
- the code (which was merged 3 years ago) should move to use the new
device tree bindings (as does tegra124 display support)

What am I missing?

Let's see what Marcel is able to do here.

Regards,
Simon