Den 04.12.2017 12.32, skrev Alexey Brodkin: > Hello, > > I'm trying to use DisplayLink USB2.0-to-HDMI adapter as the one and only > video output and I want to get Xserver working on top of that. > > I'm not very familiar with all the parts of Linux GPU/video stack > (especially its user-space counterpart) so my assumptions might be wrong > in that case please correct me. > > My first [probably incorrect] assumption is Xserver requires fbdev (/dev/fbX) > and it cannot use DRI video card natively. Is that correct? > > So to get /ded/fb0 with UDL I just enabled CONFIG_DRM_UDL & CONFIG_DRM_FBDEV_EMULATION. > That gave me boot console on HDMI screen and I was full of expectations. > But when I tried to use /dev/fb0 from whatever user-space app nothing got > displayed on the screen... as well as no error messages appeared. > > After?eyeballing at UDL code (especially in comparison with QXC which uses deferredio > as well) I noticed that in UDL fb_deferred_io_init() is called from udl_fb_open(), > i.e. .fb_open call-back (in other words every time user-space app opens /dev/fb0) > while in QXC this is done only once and much earlier in?qxlfb_create(), which is > called with .fb_probe call-back. So moved fb_deferred_io_init() in UDL driver from > udl_fb_open() to?udlfb_create() which is also called from .fb_probe. > > With that change I finally got video output via fbdev from user-space app, > but only on the first run. The next attempt to run inevitably ends with > kernel crash showing the following stack-trace (having half of the new screen > rendered on display): > ------------------------>8------------------------- > Stack Trace: > ? udl_handle_damage+0x48/0x210 > ? udl_crtc_mode_set+0x6ee/0x754 > ? drm_crtc_helper_set_mode+0x25e/0x438 > ? drm_crtc_helper_set_config+0x6d6/0x814 > ? __drm_mode_set_config_internal+0x48/0xc8 > ? drm_mode_setcrtc+0x320/0x478 > ? drm_ioctl+0x22c/0x3e4 > ? SyS_ioctl+0xa4/0x8cc > ? EV_Trap+0x108/0x10c > random: crng init done > ------------------------>8------------------------- > > I'm wondering if UDL driver (its DRM flavor) was ever tested for > fbdev in user-space? If so and it really works for somebody > maybe I'm doing something terribly wrong - in that case any comments > are very welcome. udl uses shmem buffers which doesn't work with fbdev deferred io. They fight over page->lru and page->mapping. See this commit: drm/udl: disable fb_defio by default 677d23b70bf949f75746c80cbae92c233c6b5e2a Noralf. > One interesting note: it seems like pure DRI stuff works like a charm. > I may run kmscube perfectly fine without any changes whatsoever and do it > many times in a row. > > Thanks in advance, > Alexey > _______________________________________________ > dri-devel mailing list > dri-devel at lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel >
