Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752660AbdLLV6M (ORCPT ); Tue, 12 Dec 2017 16:58:12 -0500 Received: from smtp.domeneshop.no ([194.63.252.55]:46203 "EHLO smtp.domeneshop.no" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752564AbdLLV6L (ORCPT ); Tue, 12 Dec 2017 16:58:11 -0500 Subject: Re: UDL's fbdev doesn't work for user-space apps To: Alexey Brodkin , "airlied@gmail.com" Cc: "tutankhamen@chromium.org" , "daniel.vetter@ffwll.ch" , "linux-kernel@vger.kernel.org" , "dri-devel@lists.freedesktop.org" , "airlied@redhat.com" , "linux-snps-arc@lists.infradead.org" , "hshi@chromium.org" References: <1512387175.4977.24.camel@synopsys.com> From: =?UTF-8?Q?Noralf_Tr=c3=b8nnes?= Message-ID: <370a9121-788d-a0eb-e4cb-c3b09f7bd6ce@tronnes.org> Date: Tue, 12 Dec 2017 22:58:04 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: <1512387175.4977.24.camel@synopsys.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2871 Lines: 67 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@lists.freedesktop.org > https://lists.freedesktop.org/mailman/listinfo/dri-devel >