Hi Stefan, On Friday, 7 September 2018 21:25:40 EEST Stefan Agner wrote: > On 07.09.2018 00:10, Linus Walleij wrote: > > On Thu, Sep 6, 2018 at 10:25 PM Stefan Agner <stefan@xxxxxxxx> wrote: > >> Ok, I read a bit up on the history of bridge timing, especially: > >> https://www.spinics.net/lists/dri-devel/msg155618.html > >> > >> IMHO, this got overengineered. For displays we do not need all that > >> setup/sample delay timing information, and much longer cables are in > >> use. So why is all that needed for bridges? > > > > I also avoided the overhead of creating this abstraction initially. > > > > But after doing it I have this Stockholm syndrome that I start > > liking what Laurent told me to do. > > > >> For Linus case, the THS8134(A/B) data sheet I found (revised March 2010) > >> clearly states: > >> Clock input. A rising edge on CLK latches RPr0-7, GY0-7, BPb0-7. > >> > >> So we need to drive on negative edge, hence DRM_BUS_FLAG_PIXDATA_NEGEDGE > >> should be used, which makes the pl111 driver setting TIM2_IPC: > >> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0121d/inde > >> x.html > > > > That is easy to say, but if I just set that up in code, even with a good > > comment it is hard for the next reader to understand what is going > > on. The central question will be, why does PL111 need to do this > > but not R-Car though they are using the same bridge? > > That is something I have experienced before. Depending on drive > strength, and I guess sampling delay etc, it might sample just late > enough that signals are stable and no issues are really visible on the > screen. In those cases different cabling, changes in drive strength, or > even production variations might suddenly show artefacts though... My > point being, just because it supposedly works, does not mean that it is > right. > > I did introduce the bus flags a while ago: > https://patchwork.kernel.org/patch/8254801/ > https://patchwork.kernel.org/patch/8254821/ > > And then had to debug quite some controller and display issues around > pixel clock polarity: > https://patchwork.kernel.org/patch/8117181/ > https://patchwork.kernel.org/patch/9465485/ > > E.g. in that last case, the pixel clock polarity flags were interpreted > wrong on both controller and display side. Hence this worked fine for > the case already upstream, but it did not work in my case... > > In the end, best thing is to look on the bus. Parallel display signals > are still rather slow, and usually rather easy to scope. > > PL111 needs to do this because its standard setting is driving on rising > edge. However, most displays as well as that bridge sample data on > rising edge. > > I guess R-Car DU drives on negative edge by default, but can't say for > sure without hardware/data sheet. No, it drives on the rising edge by default. > As far as I can tell the R-Car DU driver also does not use the > DRM_BUS_FLAG_PIXDATA_*EDGE flags currently. That's because the hardware default happens to work. As soon as I'll need to support a system that requires the other edge, I'll have to add the corresponding logic to the driver. So far it hasn't been needed, so I haven't bothered. > > So this elaborate model gives a better transfer of abstract concepts > > to whoever needs to touch that code next. The code is not just > > logic, but also our map of the world and the documentation of our > > problem space. > > > > Donald Knuth has this idea about literate programming which even > > turns the documentation/implementation process around. We are > > not there, not even remotely, but IMO the more complex the problem. > > the more we need to convey our thinking, not just our solution. > > Fully agree. > > My argument here is, that the map of the world (code) and the problem > are not aligned. -- Regards, Laurent Pinchart
