On Wed, Mar 23, 2022 at 09:42:11AM +0100, Max Krummenacher wrote: > Am Freitag, den 18.03.2022, 17:53 +0000 schrieb Dave Stevenson: > > On Fri, 18 Mar 2022 at 17:16, Maxime Ripard <maxime@xxxxxxxxxx> wrote: > > > On Fri, Mar 18, 2022 at 05:05:11PM +0000, Dave Stevenson wrote: > > > > Hi Maxime > > > > > > > > On Fri, 18 Mar 2022 at 16:35, Maxime Ripard <maxime@xxxxxxxxxx> wrote: > > > > > On Mon, Mar 07, 2022 at 04:26:56PM +0100, Max Krummenacher wrote: > > > > > > On Wed, Mar 2, 2022 at 5:22 PM Marek Vasut <marex@xxxxxxx> wrote: > > > > > > > On 3/2/22 15:21, Maxime Ripard wrote: > > > > > > > > Hi, > > > > > > > > > > > > > > Hi, > > > > > > > > > > > > > > > Please try to avoid top posting > > > > > > Sorry. > > > > > > > > > > > > > > On Wed, Feb 23, 2022 at 04:25:19PM +0100, Max Krummenacher wrote: > > > > > > > > > The goal here is to set the element bus_format in the struct > > > > > > > > > panel_desc. This is an enum with the possible values defined in > > > > > > > > > include/uapi/linux/media-bus-format.h. > > > > > > > > > > > > > > > > > > The enum values are not constructed in a way that you could calculate > > > > > > > > > the value from color channel width/shift/mapping/whatever. You rather > > > > > > > > > would have to check if the combination of color channel > > > > > > > > > width/shift/mapping/whatever maps to an existing value and otherwise > > > > > > > > > EINVAL out. > > > > > > > > > > > > > > > > > > I don't see the value in having yet another way of how this > > > > > > > > > information can be specified and then having to write a more > > > > > > > > > complicated parser which maps the dt data to bus_format. > > > > > > > > > > > > > > > > Generally speaking, sending an RFC without explicitly stating what you > > > > > > > > want a comment on isn't very efficient. > > > > > > > > > > > > > > Isn't that what RFC stands for -- Request For Comment ? > > > > > > > > > > > > I hoped that the link to the original discussion was enough. > > > > > > > > > > > > panel-simple used to have a finite number of hardcoded panels selected > > > > > > by their compatible. > > > > > > The following patchsets added a compatible 'panel-dpi' which should > > > > > > allow to specify the panel in the device tree with timing etc. > > > > > > > > > > > > https://patchwork.kernel.org/project/dri-devel/patch/20200216181513.28109-6-sam@xxxxxxxxxxxx/ > > > > > > In the same release cycle part of it got reverted: > > > > > > > > > > > > https://patchwork.kernel.org/project/dri-devel/patch/20200314153047.2486-3-sam@xxxxxxxxxxxx/ > > > > > > With this it is no longer possible to set bus_format. > > > > > > > > > > > > The explanation what makes the use of a property "data-mapping" not a > > > > > > suitable way in that revert > > > > > > is a bit vague. > > > > > > > > > > Indeed, but I can only guess. BGR666 in itself doesn't mean much for > > > > > example. Chances are the DPI interface will use a 24 bit bus, so where > > > > > is the padding? > > > > > > > > > > I think that's what Sam and Laurent were talking about: there wasn't > > > > > enough information encoded in that property to properly describe the > > > > > format, hence the revert. > > I agree that the strings used to set "data-mapping" weren't self explaining. > However, as there was a > clear 1:1 relation to the bus_format value the meaning > wasn't ambiguous at all. > > > > > > > > > MEDIA_BUS_FMT_RGB666_1X18 defines an 18bit bus, therefore there is no > > > > padding. "bgr666" was selecting that media bus code (I won't ask about > > > > the rgb/bgr swap). > > > > > > > > If there is padding on a 24 bit bus, then you'd use (for example) > > > > MEDIA_BUS_FMT_RGB666_1X24_CPADHI to denote that the top 2 bits of each > > > > colour are the padding. Define and use a PADLO variant if the padding > > > > is the low bits. > > > > > > Yeah, that's kind of my point actually :) > > > > Ah, OK :) > > > > > Just having a rgb666 string won't allow to differentiate between > > > MEDIA_BUS_FMT_RGB666_1X18 and MEDIA_BUS_FMT_RGB666_1X24_CPADHI: both are > > > RGB666 formats. Or we could say that it's MEDIA_BUS_FMT_RGB666_1X18 and > > > then when we'll need MEDIA_BUS_FMT_RGB666_1X24_CPADHI we'll add a new > > > string but that usually leads to inconsistent or weird names, so this > > > isn't ideal. > > We're on the same page that the strings that were used aren't self > explaining and do not follow a pattern which would make it easy to > extend. However that is something I addressed in my RFC proposal, not? > > > > > > > > The string matching would need to be extended to have some string to > > > > select those codes ("lvds666" is a weird choice from the original > > > > patch). > > > > > > > > Taking those media bus codes and handling them appropriately is > > > > already done in vc4_dpi [1], and the vendor tree has gained > > > > BGR666_1X18 and BGR666_1X24_CPADHI [2] as they aren't defined in > > > > mainline. > > > > > > > > Now this does potentially balloon out the number of MEDIA_BUS_FMT_xxx > > > > defines needed, but that's the downside of having defines for all > > > > formats. > > > > > > > > (I will admit to having a similar change in the Pi vendor tree that > > > > allows the media bus code to be selected explicitly by hex value). > > > > > > I think having an integer value is indeed better: it doesn't change much > > > in the device tree if we're using a header, it makes the driver simpler > > > since we don't have to parse a string, and we can easily extend it or > > > rename the define, it won't change the ABI. > > Fine with me. > > > > > > > I'm not sure using the raw media bus format value is ideal though, since > > > that value could then be used by any OS, and it would effectively force > > > the mbus stuff down their throat. > > I disagree here, this forces us to use code to map the device tree enum > to the kernel enum for Linux, i.e. adds complexity and maintenance work > if additional bus_formats are needed. > Assuming there is another OS which uses the device tree it would not > make a difference, that OS would still need to map the device tree enum > to the corresponding representation in their kernel. So, you don't want to do something in Linux, but would expect someone else to be completely ok with that? > I would copy the definitions of media-bus-format.h into a header in > include/dt-bindings similarly as it is done for > include/dt-bindings/display/sdtv-standards.h for TV standards. That might not be an option: that header is licensed under the GPL, device trees are usually licensed under GPL+MIT, and we don't have any requirements on the license for other projects using a DT (hence the dual license). Maxime
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