Hi Noralf On Sun, Feb 02, 2020 at 04:41:54PM +0100, Noralf Trønnes wrote: > Hi, > > Since I'm the original author of fbtft I thought I'd highlight a couple > of issues that's probably not well known. > > Right after fbtft was added, fbdev was closed for new drivers[1] and > the fbdev maintainer wanted to remove fbtft as a consequence of that > decision, but Greg KH said he'd keep fbtft drivers in staging > "until a matching DRM driver is present in the tree"[2]. > > There are 2 issues wrt the goal of making a matching DRM driver > (strictly speaking). One is impossible to do (policy), the other is > unlikely to happen: > > 1. Device Tree 'init' property > > All fbtft drivers have controller setup code that matches one > particular display panel. From the start of fbtft it was possible to > override this using platform data. Later when Device Tree support was > added, an 'init=' property to do the same was added. > > Example: > init = <0x10000e5 0x78F0 > 0x1000001 0x0100 > 0x2000032 > 0x1000007 0x0133>; > > This translates to: > register_write(0x00e5, 0x78F0); > register_write(0x0001, 0x0100); > mdelay(32); > register_write(0x0007, 0x0133); > > AFAIU setting register values from DT is a no-go, so this functionality > can't be replicated in a DRM driver. Many displays are made to work > using this mechanism, in particular ili9341 based ones. > > 2. Parallel bus interface > > All fbtft drivers support both a SPI and a parallel bus interface > through the fbtft helper module. Many (not all) controllers support more > than one interface. The parallel bus support was added to fbtft in its > early days when not many SPI displays were available (nowadays there's > lots to choose from). fbtft uses bitbanging to drive the parallel > interface so it's really slow, even more slow than SPI and SPI with DMA > beats it thoroughly. I know there are people that use the paralell bus > support, but I don't see much point in it unless we get a parallel bus > subsystem that can use the dedicated hw on certain SoC's (Beaglebone, > Pi). And those SOC's most likely have a parallel video/RGB bus as well, > which IMO is a much better option for a panel. > > > The following drivers have DRM counterparts that have the same panel > setup code: > > - fb_hx8357d.c: drivers/gpu/drm/tiny/hx8357d.c > - fb_ili9341.c: drivers/gpu/drm/tiny/mi0283qt.c > - fb_st7735r.c: drivers/gpu/drm/tiny/st7735r.c > - fb_ili9486.c: Patches are posted on dri-devel[3] > > But they don't support all panels based on that controller and they > don't have parallel bus support. > > There is actually also another obstacle for conversion and that is, some > of the displays (for which there is builtin driver support) might be > impossible to source except as second hand. And it's not always obvious > which panel is supported by a certain driver. > At least the displays supported by these drivers are listed as > discontinued on the fbtft wiki[4]: > - fb_hx8340bn.c > - fb_hx8347d.c > - fb_ili9320 > > This one never made it from a prototype to an actual product, because > it was too slow: > - fb_watterott.c > > I have no plans to convert fbtft drivers myself, but I figured a 5 year > anniversary was a good excuse for a status update. Thanks for the history lesson and the status update, a very informative and interesting read. Thanks for all your work in this area! Sam > > Noralf. > > [1] https://lkml.org/lkml/2015/9/24/253 > [2] https://lkml.org/lkml/2016/11/23/146 > [3] https://patchwork.freedesktop.org/series/72645/ > [4] https://github.com/notro/fbtft/wiki/LCD-Modules#discontinued-products > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > https://lists.freedesktop.org/mailman/listinfo/dri-devel _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
