Hi Maxime
On Fri, 18 Mar 2022 at 16:35, Maxime Ripard maxime@cerno.tech 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@denx.de 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-... In the same release cycle part of it got reverted: https://patchwork.kernel.org/project/dri-devel/patch/20200314153047.2486-3-s... 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... [2] https://github.com/raspberrypi/linux/blob/rpi-5.15.y/include/uapi/linux/medi...
The RFC revert of the revert https://patchwork.kernel.org/project/dri-devel/patch/20220201110717.3585-1-c... 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