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Subject: Re: [PATCH v3 0/7] Add tda998x (HDMI) support to atmel-hlcdc
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc:     jacopo mondi <jacopo@jmondi.org>, linux-kernel@vger.kernel.org,
        David Airlie <airlied@linux.ie>,
        Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Gustavo Padovan <gustavo@padovan.org>,
        Sean Paul <seanpaul@chromium.org>,
        Russell King <linux@armlinux.org.uk>,
        Jacopo Mondi <jacopo+renesas@jmondi.org>,
        dri-devel@lists.freedesktop.org, devicetree@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org
References: <20180419162751.25223-1-peda@axentia.se>
 <20180420113859.GL4235@w540>
 <840625e9-42ff-49b7-605a-202b4d980beb@axentia.se> <1549346.R82tLOL7eo@avalon>
From:   Peter Rosin <peda@axentia.se>
Organization: Axentia Technologies AB
Message-ID: <faed23e9-4bcd-f1f4-77cd-41044f2e7b59@axentia.se>
Date:   Sat, 21 Apr 2018 17:05:19 +0200
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On 2018-04-21 10:38, Laurent Pinchart wrote:
> Hi Peter,
> 
> On Friday, 20 April 2018 15:55:50 EEST Peter Rosin wrote:
>> On 2018-04-20 13:38, jacopo mondi wrote:
>>> On Fri, Apr 20, 2018 at 01:05:21PM +0200, Peter Rosin wrote:
>>>> On 2018-04-20 12:18, Laurent Pinchart wrote:
>>>>> On Friday, 20 April 2018 11:52:35 EEST jacopo mondi wrote:
>>>>>> Hi Peter,
>>>>>>
>>>>>> I've been a bit a pain in the arse for you recently, but please
>>>>>> bear with me a bit more, and sorry for jumping late on the band wagon.
>>>>>>
>>>>>> On Thu, Apr 19, 2018 at 06:27:44PM +0200, Peter Rosin wrote:
>>>>>>> Hi!
>>>>>>>
>>>>>>> I naively thought that since there was support for both nxp,tda19988
>>>>>>> (in the tda998x driver) and the atmel-hlcdc, things would be a smooth
>>>>>>> ride. But it wasn't, so I started looking around and realized I had to
>>>>>>> fix things.
>>>>>>>
>>>>>>> In v1 and v2 I fixed things by making the atmel-hlcdc driver a master
>>>>>>> component, but now in v3 I fix things by making the tda998x driver
>>>>>>> a bridge instead. This was after a suggestion from Boris Brezillion
>>>>
>>>> That should be Brezillon, sorry for being sloppy with the spelling.
>>>>
>>>>>>> (the atmel-hlcdc maintainer), so there was some risk of bias ... but
>>>>>>> after comparing what was needed, I too find the bridge approach
>>>>>>> better.
>>>>>>>
>>>>>>> In addition to the above, our PCB interface between the SAMA5D3 and
>>>>>>> the HDMI encoder is only using 16 bits, and this has to be described
>>>>>>> somewhere, or the atmel-hlcdc driver have no chance of selecting the
>>>>>>> correct output mode. Since I have similar problems with a ds90c185
>>>>>>> lvds encoder I added patches to override the atmel-hlcdc output format
>>>>>>> via DT properties compatible with the media video-interface binding
>>>>>>> and things start to play together.
>>>>>>>
>>>>>>> Since this series superseeds the bridge series [1], I have included
>>>>>>> the leftover bindings patch for the ti,ds90c185 here.
>>>>>>
>>>>>> I feel like this series would look better if it would make use of the
>>>>>> proposed bridge media bus format support I have recently sent out [1]
>>>>>> (and which was not there when you first sent v1).
>>>>>>
>>>>>> I understand your fundamental problem here is that the bus format
>>>>>> that should be reported by your bridge is different from the ones
>>>>>> actually supported by the TDA19988 chip, as the wirings ground some
>>>>>> of the input pins.
>>>>>>
>>>>>> Although this is defintely something that could be described in the
>>>>>> bridge's own OF node with the 'bus_width' property, and what I would
>>>>>> do, now that you have made a bridge out from the tda19988 driver, is:
>>>>>>
>>>>>> 1) Set the bridge accepted input bus_format parsing its pin
>>>>>> configuration, or default it if that's not implemented yet.
>>>>>> This will likely be rgb888. You can do that using the trivial
>>>>>> support for bridge input image formats implemented by my series.
>>>>>> 2) Specify in the bridge endpoint a 'bus_width' of <16>
>>>>>> 3) In your atmel-hlcd get both the image format of the bridge (rgb888)
>>>>>> and parse the remote endpoint property 'bus_width' and get the <16>
>>>>>> value back.
>>>>>
>>>>> Parsing properties of remote nodes should be avoided as much as
>>>>> possible, as they need to be interpreted in the context of the DT
>>>>> bindings related to the compatible string applicable to that node. I'd
>>>>> rather have the bus_width property in the local endpoint node.
>>>>
>>>> In addition to that, my view of this binding
>>>>
>>>> 	endpoint {
>>>> 		bus-type = <0>;
>>>
>>> bus-type is used by v4l2_fwnode helpers to decide which type of bus
>>> they're dealing with iirc. Here it seems to me it has no added
>>> value, also because in your bindings description "it shall be 0" and
>>> it is not parsed anywhere in you code, but no big deal....
>>
>> bus-type is indeed parsed and verified to be zero which means auto-detect
>> according to the video-interfaces binding. An auto-detect bus-type with
>> a given bus-width means a parallel interface IIUC.
> 
> I believe you could leave it out though. Your display controller only supports 
> parallel buses, so there's no need to specify the bus type explicitly.

You need it if you want to verify that you conform to the video-interfaces
binding. And that verification is only needed *if* we want to build upon
the drm_of_media_bus_fmt function so that it eventually support other formats.

Where I'm coming from is this discussion with Rob:
https://lkml.org/lkml/2018/4/9/286

I interpreted that as is seen in patch 2/7 and 3/7 in v3 (and v2). True,
I might have read too much into his desire to have something generic,
and that the generic bit only applied to the binding, but 2/7 and 3/7
was how I interpreted that conversation.

>> From patch 3/7:
>>
>> 	if (of_property_read_u32(node, "bus-type", &bus_type))
>> 		return 0;
>> 	if (bus_type != 0)
>> 		return -EINVAL;
>>
>>>> 		bus-widht = <16>;
>>>> 	};
>>>>
>>>> is that it always means rgb565. See further below.
>>>>
>>>>>> 4) Set the correct format in the atmel hlcd driver to accommodate a
>>>>>> bridge that wants rgb888 on a 16 bit wide bus (that would be rgb565,
>>>>>> or are there other possible combinations I am missing?)
>>>>>>
>>>>>> I would consider this better mostly because in this series you are
>>>>>> creating a semantic for the whole DRM subsystem on the 'bus_width'
>>>>>> property, where numerical values as '12', '16' etc are arbitrary tied
>>>>>> to the selection of a media bus format. At least you should use a
>>>>>> common set of defines [1] between the device tree and the driver,
>>>>>> but this looks anyway fragile imho.
>>>>>
>>>>> This I agree with though. Combining the remote bus format with the local
>>>>> bus width should fix the problem without having to parse remote
>>>>> properties.
>>>>
>>>> My thinking was that the binding with bus-type = <0> and bus-width =
>>>> <bpp> would mean a parallel bus (type 0 means auto-detect and with a bus-
>>>> width that auto-detect means a parallel bus) and the most natural/common
>>>> interpretation of that bus-width. For bus widths of 12, 16, 18, 24, 30
>>>> etc I think that would be rgb444, rgb565, rgb666, rgb888, rgb101010 (or,
>>>> I'm first so I get to define the default). If you have some other
>>>> interpretation of a bus with that width, you'd need to extend the
>>>> video-interface binding with some way of saying what you need, perhaps
>>>> using some kind of data mapping or something to say e.g. bgr666. And
>>>> you'd need some kind of indicator if you have YUV signals instead of
>>>> RGB, and LVDS isn't a completely parallel bus, so you'd need something
>>>> for that. Etc.
>>>
>>> The fundamental issue here is that you're tying the bus with to an
>>> image format. Why <16> can't be, say, MEDIA_BUS_FMT_YVYU8_1X16? it
>>> spans to 16 bits, doesn't it? And I'm sorry but the 'I'm first so I
>>> get to define defaults' argument doesn't apply here.
>>
>> I never said that <16> could not be something YUV. I said that <16> without
>> any further information is RGB. But you are right, the fundamental issue
>> is that I want to specify the full format in the endpoint node, while you
>> do not for some reason I don't understand. And maybe saying that "<16>
>> without more info is RGB" is too presumptuous, but then I think that the
>> right fix is adding something clearly indicating RGB is the way to go, so
>> that there is a way for endpoints to specify RGB565 (or whatever is needed).
> 
> I think that would lack genericity though. I could easily imagine a setup 
> similar to yours where the bridge can accept different types of parallel 
> formats (RGB, BGR, YUV, ...). While the bus width is a property of the PCB 
> routing and is thus fixed, the format wouldn't be a property of the platform 
> but would be dynamic. I believe that combining the bus-width DT property that 
> carries static platform information with the dynamic format reported by the 
> bridge (through the API proposed by Jacopo) would then be a more generic 
> approach.

This is a valid argument.

> You don't have to depend on Jacopo's patch series though, I would be totally 
> fine with assuming that a 16-bit width means RGB565 in the atmel-hlcdc driver. 
> What I'm not comfortable with is hardcoding that assumption in the helper 
> defined in patch 3/7. I would thus propose moving that code to the atmel-hlcdc 
> driver for now, and creating a generic helper that uses both bus-width and the 
> format queried from the bridge when Jacopo's series makes it to mainline. 
> Would that be an acceptable approach for you ?

I just want something acceptable that lets me set a few bits in a damn
register to my liking, ok? :-)

I'll re-spin with the meat of drm_of_media_bus_fmt moved to a static
function in the atmel-hlcdc driver (and squashed with 4/7).

Cheers,
Peter