Den 02.02.2022 16.04, skrev Pekka Paalanen: > On Wed, 2 Feb 2022 10:45:42 +0100 > Noralf Trønnes <noralf@xxxxxxxxxxx> wrote: > >> Den 02.02.2022 10.14, skrev Thomas Zimmermann: >>> Hi Noralf, >>> >>> since you're here, I'll just hijack the discussion to ask something only >>> semi-related. >>> >>> IIRC the gud driver doesn't update the display immediately during atomic >>> commits. Instead, it instructs a helper thread to do the update. What's >>> the rational behind this design? Is that something we should adopt for >>> other drivers that operate over slow buses (USB, I2C, etc)? Would this >>> be relevant for the ssd1307 driver? >>> >> >> Async flushing is only necessary on multi display setups where there's >> only one rendering loop for all the displays. I saw what tiny/gm12u320.c >> did and Hans gave me the rationale. The SPI drivers run flushing inline. >> Info on the gud wiki: >> https://github.com/notro/gud/wiki/Linux-Host-Driver#asynchronous-flushing > > Hi, > > please also consider that userspace may throttle to the KMS pageflip > events. If the pageflip event is immediate from submitting a flip, that > could mean userspace will be repainting in a busy-loop, like 1 kHz. > However, I remember something about virtual KMS drivers doing exactly > this, and there being something that tells userspace to throttle itself > instead of depending on pageflip completions. I just forget how that is > supposed to work, and I'm fairly sure that e.g. Weston does not behave > well there. > > Unfortunately, the pageflip event is also what synchronises FB usage. > Once flipping in a new FB completed, the old FB is free for re-use. > But, if the kernel is still copying out from the old FB, userspace may > partially overwrite the contents, temporarily leading to an incomplete > or too new image on screen. Do you have anything to prevent that? > Unfortunately not. One solution would be to make a buffer copy during the flip and do the USB transfer async but I haven't looked into that. My plan is to wait and see what problems users report back before trying to fix anything. Noralf.
