Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759813AbcDMO7g (ORCPT ); Wed, 13 Apr 2016 10:59:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49773 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754526AbcDMO7L (ORCPT ); Wed, 13 Apr 2016 10:59:11 -0400 From: Lyude To: intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, Jani Nikula , =?UTF-8?q?Ville=20Syrj=C3=A4l=C3=A4?= Cc: Lyude , David Airlie , linux-kernel@vger.kernel.org (open list) Subject: [PATCH v5 3/4] drm/dp_helper: Perform throw-away read before actual read in drm_dp_dpcd_read() Date: Wed, 13 Apr 2016 10:58:32 -0400 Message-Id: <1460559513-32280-4-git-send-email-cpaul@redhat.com> In-Reply-To: <1460559513-32280-1-git-send-email-cpaul@redhat.com> References: <87d1pvt9ka.fsf@intel.com> <1460559513-32280-1-git-send-email-cpaul@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1490 Lines: 44 This is part of a patch series to migrate all of the workarounds for commonly seen behavior from bad sinks in intel_dp_dpcd_read_wake() to drm's DP helper. Some sinks will just return garbage for the first aux tranaction they receive when coming out of sleep mode, so we need to perform an additional read before the actual read to workaround this. Changes since v5 - If the throwaway read in drm_dp_dpcd_read() fails, return the error from that instead of continuing. This follows the same logic we do in drm_dp_dpcd_access() (e.g. the error from the first transaction may differ from the errors that proceeding attempts might return). Signed-off-by: Lyude --- drivers/gpu/drm/drm_dp_helper.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c index 540c3e4..aa80702 100644 --- a/drivers/gpu/drm/drm_dp_helper.c +++ b/drivers/gpu/drm/drm_dp_helper.c @@ -248,6 +248,18 @@ unlock: ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset, void *buffer, size_t size) { + int ret; + + /* + * Sometimes we just get the same incorrect byte repeated over the + * entire buffer. Doing one throw away read initially seems to "solve" + * it. + */ + ret = drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, DP_DPCD_REV, buffer, + 1); + if (ret != 1) + return ret; + return drm_dp_dpcd_access(aux, DP_AUX_NATIVE_READ, offset, buffer, size); } -- 2.5.5