Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752271AbdF0Gxm (ORCPT ); Tue, 27 Jun 2017 02:53:42 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:44856 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752020AbdF0Gxf (ORCPT ); Tue, 27 Jun 2017 02:53:35 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 90714600C9 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=architt@codeaurora.org Subject: Re: [PATCH 1/7] drm/bridge: Support hotplugging panel-bridge. To: Eric Anholt , Boris Brezillon , Laurent Pinchart Cc: Benjamin Gaignard , Mark Rutland , devicetree@vger.kernel.org, Linux Kernel Mailing List , "dri-devel@lists.freedesktop.org" , Philippe Cornu , Rob Herring , Thierry Reding References: <20170615204130.19255-1-eric@anholt.net> <20170615204130.19255-2-eric@anholt.net> <777ee1b1-e5ce-ea29-8a48-f792354a22d1@codeaurora.org> <871sqkouvr.fsf@eliezer.anholt.net> <8e148170-b626-b426-3c94-b93d2746f4ce@codeaurora.org> <871sqe7ei0.fsf@eliezer.anholt.net> <20170622112332.7ec7b65c@bbrezillon> <87o9te1igt.fsf@eliezer.anholt.net> From: Archit Taneja Message-ID: <28e370bb-cb81-a564-6a17-0253b95f5370@codeaurora.org> Date: Tue, 27 Jun 2017 12:23:25 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: <87o9te1igt.fsf@eliezer.anholt.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5230 Lines: 102 On 06/24/2017 03:20 AM, Eric Anholt wrote: > Boris Brezillon writes: > >> On Thu, 22 Jun 2017 13:47:43 +0530 >> Archit Taneja wrote: >> >>> On 06/22/2017 01:20 PM, Benjamin Gaignard wrote: >>>> 2017-06-20 19:31 GMT+02:00 Eric Anholt : >>>>> Archit Taneja writes: >>>>> >>>>>> On 06/16/2017 08:13 PM, Eric Anholt wrote: >>>>>>> Archit Taneja writes: >>>>>>> >>>>>>>> On 06/16/2017 02:11 AM, Eric Anholt wrote: >>>>>>>>> If the panel-bridge is being set up after the drm_mode_config_reset(), >>>>>>>>> then the connector's state would never get initialized, and we'd >>>>>>>>> dereference the NULL in the hotplug path. We also need to register >>>>>>>>> the connector, so that userspace can get at it. >>>>>>>>> >>>>>>>> >>>>>>>> Shouldn't the KMS driver make sure the panel-bridge is set up before >>>>>>>> drm_mode_config_reset? Is it the case when we're inserting the >>>>>>>> panel-bridge driver as a module? >>>>>>>> >>>>>>>> >>>>>>>> All the connectors that have been added are registered automatically >>>>>>>> when drm_dev_register() is called by the KMS driver. Registering a >>>>>>>> connector in the middle of setting up our driver is prone to race >>>>>>>> conditions if the userspace decides to use them immediately. >>>>>>> >>>>>>> Yeah, this is fixing initializing panel_bridge at DSI host_attach time, >>>>>>> which in the case of a panel module that creates the DSI device >>>>>>> (adv7533-style, like you said I should use as a reference) will be after >>>>>>> drm_mode_config_reset() and drm_dev_register(). >>>>>> >>>>>> Okay. In the case of the msm kms driver, we defer probe until the >>>>>> adv7533 module is inserted, only then we proceed to drm_mode_config_reset() >>>>>> and drm_dev_register(). I assumed this was the general practice followed by >>>>>> most kms drivers. I.,e the kms driver defers probe until all connector >>>>>> related modules are inserted, and only then proceed to create a drm device. >>>>> >>>>> The problem, though, is the panel driver needs the MIPI DSI host to >>>>> exist to call mipi_dsi_device_register_full() during the probe process. >>>>> The adv7533 driver gets around this by registering the DSI device in the >>>>> bridge attach step, but drm_panel doesn't have an attach step. >>> >>> I'm not sure how we can get around this. We had discussion about this on irc >>> recently, but couldn't come up with a good conclusion. We could come up with a >>> panel_attach() callback to make it similar to bridges, but that's just us avoiding >>> the real issue. >> >> How about making DSI dev registration fully asynchronous, that is, DSI >> devs declared in the DT under the DSI host node will be >> registered/attached at probe time, and devs using another control bus >> (like the adv7533 controller over i2c) will be registered afterwards. >> >> That implies moving the drm_brige registration logic outside of the DSI >> host ->probe() path. The idea would be to check if all devs connected >> to the DSI bus are ready at dsi_host->attach() time. If they are, we >> can finally register the XXX -> DSI bridge. If they're not (because >> some devs connected to the DSI bus have not been probed yet), then we >> do not register the drm_bridge and wait for the next dsi_host->attach() >> event. >> >> With this solution, I think we can avoid the chicken&egg problem where >> the adv7533 dev is waiting for the DSI host to be probed to be able to >> register a DSI dev with mipi_dsi_device_register_full() and the DSI >> host needs all devs to be registered to not return -EPROBE_DEFER. > > I've now tried having mipi_dsi_device_register_full() succeed early and > just do the device-add part when the host shows up. The problem is > there's still mipi_dsi_attach(), which needs to be delayed until the > panel driver fills in the rest of mipi_dsi_device's fields. Why aren't > those part of the info? In the past, the only way to create mipi_dsi_devices was to add them as children DT nodes under the DSI host node. The KMS driver calls mipi_dsi_host_register(), which resulted in the creation of all the children mipi_dsi_devices. The DSI host was still unaware of parameters like number of lanes, mode_flags etc. mipi_dsi_attach() was a way by which the DSI device could share these with its DSI host. Another use of mipi_dsi_attach()/detach() is if the panel driver wants to notify the DSI host an update in one of the device's fields (for example, a change in the number of lanes). We've recently added another way of creating mipi_dsi_devices (i.e, by making a driver call mipi_dsi_device_register_full) for cases where there isn't a DT representation of the DSI device. For the second method, the need to call mipi_dsi_attach() separately becomes sort of redundant. We could consider embedding the other mipi_dsi_device fields (lanes etc) in mipi_dsi_device_info itself, and invoke the DSI host's ->attach() callback through mipi_dsi_device_register_full() itself. Would that help resolving your issue? Archit -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project