Re: [RFC 0/1] drm: Add Grain Media GM12U320 kms driver

["Noralf Trønnes" <noralf@tronnes.org>]

Den 07.06.2017 22.19, skrev Noralf Trønnes:
>
> Den 07.06.2017 21.50, skrev Marco Diego Aurélio Mesquita:
>> On Wed, Jun 7, 2017 at 4:38 PM, Noralf Trønnes <noralf@tronnes.org> 
>> wrote:
>>> Den 07.06.2017 20.46, skrev Marco Diego Aurélio Mesquita:
>>>> Hi Noralf!
>>>>
>>>> On Wed, Jun 7, 2017 at 1:56 PM, Noralf Trønnes <noralf@tronnes.org> 
>>>> wrote:
>>>>> tinydrm is targeted at displays on slow busses where userspace tells
>>>>> the driver which part of the framebuffer it has changed to minimize
>>>>> the transfer.
>>>>>
>>>>> The PL111 driver uses drm_simple_display_pipe and might be a better
>>>>> example for your use case:
>>>>> https://cgit.freedesktop.org/drm/drm-misc/tree/drivers/gpu/drm/pl111
>>>> I'll investigate how to modify the pl111 driver to fit my needs then.
>>>> Do you think is there any change that must be done to the
>>>> pl111_get_panel function regarding the call to
>>>> of_graph_get_next_endpoint? I mean, the udl driver (which the gm12u320
>>>> driver is based on) does not have a panel; would any change be needed
>>>> because of this?
>>>
>>> I guess you can keep your get_modes and do something like this:
>>>
>>> static const struct drm_connector_helper_funcs gm12u320_helper_funcs 
>>> = {
>>>      .get_modes = gm12u320_get_modes,
>>> };
>>>
>>> .best_encoder has a default that fits your 1 encoder case.
>>>
>>> static const struct drm_connector_funcs gm12u320_connector_funcs = {
>>>      .fill_modes = drm_helper_probe_single_connector_modes,
>>>      .destroy = gm12u320_connector_destroy,
>>>      .detect = gm12u320_detect,
>>>      .dpms = drm_atomic_helper_connector_dpms,
>>>      .reset = drm_atomic_helper_connector_reset,
>>>      .atomic_duplicate_state = 
>>> drm_atomic_helper_connector_duplicate_state,
>>>      .atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
>>> };
>>>
>>> Then you init and add helper:
>>>      drm_connector_init(dev, connector, &gm12u320_connector_funcs,
>>>                 DRM_MODE_CONNECTOR_Unknown);
>>>      drm_connector_helper_add(connector, &gm12u320_helper_funcs);
>>>
>>> And finally attach the connector to the pipe:
>>>      drm_simple_display_pipe_init(..., connector);
>>>
>>> The connector is registered when the drm device is registered.
>>>
>> Thanks Noralf! That's exactly what I did and I'll test it soon. The
>> parts that scare me the most are the calls to
>> of_graph_get_next_endpoint, of_property_read_u32_array and of_node_put
>> (in display.c, for example). I'm still not sure those will work.
>
> You don't need those since you don't have a panel nor those pads:
> https://cgit.freedesktop.org/drm/drm-misc/tree/Documentation/devicetree/bindings/display/arm,pl11x.txt 
>
>
> The reason I pointed you to the PL111 driver is because it's very simple,
> so it's easy to see how the simple pipeline works.
> It uses CMA memory and if you can use that all is well, but if you need
> to access the framebuffer from the cpu, it's not so good, because cma
> memory has uncached reads.
> The udl driver handles memory in a different way.
>
> I'm out of my depths here with memory and GEM and stuff.
>

I see that you access the framebuffer in gm12u320_copy_fb_to_blocks().
That will be be very slow with CMA memory. I have worked around this in
tinydrm by caching one line at a time to speed it up. I don't know if
this will work for you. tinydrm displays are very small so the overhead
isn't big compared to the transfer speed.
See http://elixir.free-electrons.com/linux/latest/ident/tinydrm_swab16

Maybe you can cache one cacheline size at a time? After all, you have to
pull it through the cpu cache. But as I said, memory isn't my expertise.

It greatly simplifies your driver if you can use the cma library though.
If so, maybe you even can use tinydrm with a connector of your own. You
can just make your own version of tinydrm_display_pipe_init().

Noralf.

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