From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B358EC11F69 for ; Thu, 1 Jul 2021 11:30:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9BE6661467 for ; Thu, 1 Jul 2021 11:30:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236214AbhGALdH (ORCPT ); Thu, 1 Jul 2021 07:33:07 -0400 Received: from srv6.fidu.org ([159.69.62.71]:53222 "EHLO srv6.fidu.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236130AbhGALdG (ORCPT ); Thu, 1 Jul 2021 07:33:06 -0400 Received: from localhost (localhost.localdomain [127.0.0.1]) by srv6.fidu.org (Postfix) with ESMTP id 3029FC80092; Thu, 1 Jul 2021 13:30:35 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at srv6.fidu.org Received: from srv6.fidu.org ([127.0.0.1]) by localhost (srv6.fidu.org [127.0.0.1]) (amavisd-new, port 10024) with LMTP id bFWuLJsDeuN6; Thu, 1 Jul 2021 13:30:34 +0200 (CEST) Received: from [192.168.176.112] (host-88-217-226-44.customer.m-online.net [88.217.226.44]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)) (No client certificate requested) (Authenticated sender: wse@tuxedocomputers.com) by srv6.fidu.org (Postfix) with ESMTPSA id 199D3C8008E; Thu, 1 Jul 2021 13:30:33 +0200 (CEST) Subject: Re: [PATCH v4 03/17] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property To: Pekka Paalanen Cc: sunpeng.li@amd.com, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, airlied@linux.ie, amd-gfx@lists.freedesktop.org, tzimmermann@suse.de, rodrigo.vivi@intel.com, alexander.deucher@amd.com, christian.koenig@amd.com References: <20210618091116.14428-1-wse@tuxedocomputers.com> <20210618091116.14428-4-wse@tuxedocomputers.com> <18bbd0cf-4c37-ce9d-eb63-de4131a201e1@tuxedocomputers.com> <11cd3340-46a1-9a6a-88f5-95c225863509@tuxedocomputers.com> <20210630112141.319f67eb@eldfell> <20210701104256.247538e1@eldfell> From: Werner Sembach Message-ID: Date: Thu, 1 Jul 2021 13:30:32 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210701104256.247538e1@eldfell> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 01.07.21 um 09:42 schrieb Pekka Paalanen: > On Wed, 30 Jun 2021 11:42:10 +0200 > Werner Sembach wrote: > >> Am 30.06.21 um 10:21 schrieb Pekka Paalanen: >>> On Tue, 29 Jun 2021 13:02:05 +0200 >>> Werner Sembach wrote: >>> >>>> Am 28.06.21 um 19:03 schrieb Werner Sembach: >>>>> Am 18.06.21 um 11:11 schrieb Werner Sembach: >>>>>> Add a new general drm property "active bpc" which can be used by graphic >>>>>> drivers to report the applied bit depth per pixel back to userspace. >>>>>> >>>>>> While "max bpc" can be used to change the color depth, there was no way to >>>>>> check which one actually got used. While in theory the driver chooses the >>>>>> best/highest color depth within the max bpc setting a user might not be >>>>>> fully aware what his hardware is or isn't capable off. This is meant as a >>>>>> quick way to double check the setup. >>>>>> >>>>>> In the future, automatic color calibration for screens might also depend on >>>>>> this information being available. >>>>>> >>>>>> Signed-off-by: Werner Sembach >>>>>> --- >>>>>> drivers/gpu/drm/drm_connector.c | 51 +++++++++++++++++++++++++++++++++ >>>>>> include/drm/drm_connector.h | 8 ++++++ >>>>>> 2 files changed, 59 insertions(+) >>>>>> >>>>>> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c >>>>>> index da39e7ff6965..943f6b61053b 100644 >>>>>> --- a/drivers/gpu/drm/drm_connector.c >>>>>> +++ b/drivers/gpu/drm/drm_connector.c >>>>>> @@ -1197,6 +1197,14 @@ static const struct drm_prop_enum_list dp_colorspaces[] = { >>>>>> * drm_connector_attach_max_bpc_property() to create and attach the >>>>>> * property to the connector during initialization. >>>>>> * >>>>>> + * active bpc: >>>>>> + * This read-only range property tells userspace the pixel color bit depth >>>>>> + * actually used by the hardware display engine on "the cable" on a >>>>>> + * connector. The chosen value depends on hardware capabilities, both >>>>>> + * display engine and connected monitor, and the "max bpc" property. >>>>>> + * Drivers shall use drm_connector_attach_active_bpc_property() to install >>>>>> + * this property. >>>>>> + * >>>>> Regarding "on the cable" and dithering: As far as I can tell, what the dithering option does, is setting a hardware >>>>> register here: >>>>> >>>>> - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4534 >>>>> >>>>> - https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4571 >>>>> >>>>> So dithering seems to be calculated by fixed purpose hardware/firmware outside of the driver? >>>>> >>>>> The Intel driver does not seem to set a target bpc/bpp for this hardware so I guess it defaults to 6 or 8 bpc? >>>> Never mind it does. This switch-case does affect the dithering output: >>>> https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/i915/display/intel_display.c#L4537 >>> Hi, >>> >>> I obviously do not know the intel driver or hardware at all, but >>> to me that just looks like translating from bits per pixel to bits per >>> channel in RGB mapping? >> No, if i understand the documentation correctly: Writing bit depth here >> with dithering enabled sets the dithering target bpc. >>> >>>> As found in this documentation p.548: >>>> https://01.org/sites/default/files/documentation/intel-gfx-prm-osrc-lkf-vol02c-commandreference-registers-part2.pdf >>>> >>>> So max bpc and active bpc are affecting/affected by the bpc after dithering. >>> By definition, if the cable carries N bpc, then dithering does not >>> change that. The cable still carries N bpc, but due to spatial or >>> temporal dithering, the *observed* color resolution may or may not be >>> higher than the cable bpc. >> Yes, and max bpc and active bpc tell the cable bpc ist not the >> *observed* bpc. >>> Of course, if the cable bpc is 8, and dithering targets 6 bpc, then 2 >>> LSB on the cable are always zero, right? >> I would assume that in this case only 6 bpc are actually send? Isn't the >> whole thing of dithering that you can't send, for example, 8 bpc? >>> Maybe one would want to do that if the monitor has a 6 bit panel and it >>> simply ignored the 2 LSB, and the cable cannot go down to 6 bpc. >> Is there dithering actually doing this? aka is my assumption above wrong? >> >> AMD code that confused me before, is hinting that you might be right: >> https://elixir.bootlin.com/linux/v5.13/source/drivers/gpu/drm/amd/display/dc/dce/dce_transform.c#L826 >> >> there is a set_clamp depth and a separate DCP_SPATIAL_DITHER_DEPTH_30BPP >> >>> So, what does "max bpc" mean right now? >>> >>> It seems like dither on/off is insufficient information, one would also >>> need to control the dithering target bpc. I suppose the driver has a >>> policy on how it chooses the target bpc, but what is that policy? Is >>> the dither target bpc the cable bpc or the sink bpc? >>> >>> Needless to say, I'm quite confused. >> ... We need someone who knows what dithering on intel and amd gpu >> actually means. >> >> But I don't want this to become a blocker for this patchset, because if >> there is no dithering, which seems to be the norm, the active bpc >> property is already really usefull as it is. So add a note to the docs >> that the value might be invalid when dithering is active for now? > Hi, > > not necessarily invalid. It all depends on how "max bpc" and "active > bpc" are defined. > > If they are defined and implemented as "on the cable", then they are > both well-defined and always valid, regardless of what dithering or bit > clamping does, so this is the semantics I'd would prefer. It's clear, > but of course does not tell full story. > > When better properties for dithering are added, those can then define > how it works on top of cable bpc, with no impact on "max bpc" or > "active bpc" properties. > > So if we cannot tell what "max bpc" is, then "active bpc" should just > be defined as the same thing as "max bpc" affects, and leave the > precise definition of both for later. But as long as I don't know exactly how dithering is affected by max bpc I can't tell for sure if active bpc is acting the same. That's why I wrote it is "undefined" to not run into a trap where the actual behavior have to change after the fact. > > If the definition was observed bpc, then we would have problems and > would need to know everything right now. But you can't really make > promises of observed bpc anyway, because you don't know what the > monitor does to the video signal I suppose. Unless you define it "as if > observed through an ideal theoretical monitor" which then gets awkward > to explain. Yes, that's why I think describing the "raw" signal is the best and leave it to the user to know what his or her Monitor is making out of it. > > > Thanks, > pq