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. 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. 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). Dave [1] https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/vc4/vc4_dpi.c#L154 [2] https://github.com/raspberrypi/linux/blob/rpi-5.15.y/include/uapi/linux/media-bus-format.h#L49 > > The RFC revert of the revert > > https://patchwork.kernel.org/project/dri-devel/patch/20220201110717.3585-1-cniedermaier@xxxxxxxxxxxxxxxxxx/ > > tried to get feedback what would be a way forward. This RFC tries the > > same by giving a possible solution should > > the property name and/or the a bit short strings of the original be > > the reason why it is not suitable. > > > > So the requested comments would be about what was not good enough with > > 'data-mapping' and what would be a way forward. > > > > Especially since in my limited view it is not clear why in panel-lvds > > 'data-mapping' is used to state how the bits representing colour are > > mapped to the 21 or 28 possible bit position in the LVDS lanes vs. > > here where we want to say how the bits representing colour are mapped > > to the 16/18/24 lines of the parallel interface would need a different > > binding pattern. > > There's only a few data format in LVDS, so it's ok. A DPI interface is > much more free-form, so you need to be a bit more accurate than what is > done for LVDS. > > Maxime
