Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754231AbdLDLdB (ORCPT ); Mon, 4 Dec 2017 06:33:01 -0500 Received: from smtprelay2.synopsys.com ([198.182.60.111]:52255 "EHLO smtprelay.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751720AbdLDLdA (ORCPT ); Mon, 4 Dec 2017 06:33:00 -0500 From: Alexey Brodkin To: "airlied@gmail.com" CC: "airlied@redhat.com" , "dri-devel@lists.freedesktop.org" , "hshi@chromium.org" , "daniel.vetter@ffwll.ch" , "tutankhamen@chromium.org" , "linux-snps-arc@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: UDL's fbdev doesn't work for user-space apps Thread-Topic: UDL's fbdev doesn't work for user-space apps Thread-Index: AQHTbPOhbJ0JAUja+kaaHKzVj8YXjg== Date: Mon, 4 Dec 2017 11:32:57 +0000 Message-ID: <1512387175.4977.24.camel@synopsys.com> Accept-Language: en-US, ru-RU Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.225.15.245] Content-Type: text/plain; charset="utf-8" Content-ID: <11BEB5F04B18E143B7485EBFE1DC5549@internal.synopsys.com> MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from base64 to 8bit by nfs id vB4BX7i4009038 Content-Length: 2335 Lines: 53 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. 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