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Tue, 5 May 2026 07:45:43 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id OoOFNyeg+Wk5UgAAD6G6ig (envelope-from ); Tue, 05 May 2026 07:45:43 +0000 Message-ID: <125fa5ce-e0ad-4596-98ea-f894f6a6c018@suse.de> Date: Tue, 5 May 2026 09:45:43 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for drm-misc-fixes v5 2/4] drm/hisilicon/hibmc: fix no showing when no connectors connected To: Yongbang Shi , dmitry.baryshkov@oss.qualcomm.com, tiantao6@hisilicon.com, xinliang.liu@linaro.org, maarten.lankhorst@linux.intel.com, mripard@kernel.org, airlied@gmail.com, daniel@ffwll.ch, kong.kongxinwei@hisilicon.com Cc: liangjian010@huawei.com, chenjianmin@huawei.com, fengsheng5@huawei.com, helin52@h-partners.com, shenjian15@huawei.com, shaojijie@huawei.com, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <20260423063233.1267631-1-shiyongbang@huawei.com> <20260423063233.1267631-3-shiyongbang@huawei.com> <27864583-4211-4553-bdc0-42dadd25d212@suse.de> <057ebc30-f103-4d24-b5be-bbc9b79050e8@suse.de> Content-Language: en-US From: Thomas Zimmermann Autocrypt: addr=tzimmermann@suse.de; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Flag: NO X-Spam-Score: -4.30 X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; ARC_NA(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_TWELVE(0.00)[17]; FREEMAIL_TO(0.00)[huawei.com,oss.qualcomm.com,hisilicon.com,linaro.org,linux.intel.com,kernel.org,gmail.com,ffwll.ch]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_TLS_ALL(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:mid,suse.com:url,imap1.dmz-prg2.suse.org:helo] X-Spam-Level: Hi, sorry for the late reply. I'm fairly busy. Am 28.04.26 um 14:53 schrieb Yongbang Shi: >> Hi >> >> Am 28.04.26 um 05:58 schrieb Yongbang Shi: >> [...] >>>> There's a problem with the overall logic here: if the physical status is 'connected' and the helper could not retrieve >>>> any modes, the helper should return 0 here. Then the DRM helpers do the right thing with setting up a few modes or an >>>> EDID override as a fallback. See [2]. For example, on my broken test system, I'd be able to provide my display's EDID >>>> and get the correct output. >>>> >>>> [2] https://elixir.bootlin.com/linux/v7.0.1/source/drivers/gpu/drm/drm_probe_helper.c#L436 >>>> >>>> Any code below is BMC setup and should run in an else branch, just like in ast. >>>> >>> The `drm_edid_override_connector_update` interface is used to retrieve an overridden EDID from debugfs or from firmware: >>>    * Debugfs: Requires the user to manually perform file operations to configure it; >>>    * Firmware: Requires the distribution’s operating system or the user to place the EDID binary file in the >>>                `/lib/firmware/` directory, and the `drm.edid_firmware` parameter must be specified in the GRUB boot >>>                parameters; >>> >>> The modification method you provided allows for display even when the EDID cannot be retrieved. But both of these >>> methods require cooperation from the user or the distribution's operating system, making implementation relatively >>> difficult. >> Yes, it's the established way for users to override the EDID. >> >>> Our use case requires that the KVM's display functionality remain intact even when no VGA or DisplayPort monitor is >>> connected. I believe setting `drm_set_preferred_mode` (1024x768) as the default resolution when no monitor is present >>> would be a more universal solution. Similarly, on your test system, this approach would ensure basic display >>> functionality. >> If no modes could be detected on the VGA and no EDID has been provided by the user, DRM helpers will install a set of >> default modes that are suitable for graphics cards. See [1]. The modes installed for the KVM are not all compatible with >> VGA.  If you rely on DRM helpers, you'll always get correct modes for VGA and KVM. >> >> Generally speaking, the decision of VGA-vs-KVM should be in ->detect. Having phys_state/phys_status does this very well. >> The ->get_modes function should then use whatever has been detected. >> >> [1] https://elixir.bootlin.com/linux/v7.0.1/source/drivers/gpu/drm/drm_probe_helper.c#L653 >> > Yes, I understand what you mean. But there's another issue here; we've also tested the scenario where we directly return > 0. However, we found that only three resolutions remain supported: 1024*768, 800*600 and 640*480. This is because the > resolution added via `drm_add_modes_noedid` in the helper has a maximum of 1024*768. This doesn't look good—there are > too few resolutions available for users to choose from in KVM. > > With the current implementation, when `count == 0`, we set the resolution to the highest resolution supported by our > CRTC, allowing users more options. But why do you second-guess the results from ->detect?  Th call to ->detect already established that there's a display connected.  If ->get_modes cannot find useful modes, then don't treat it as a BMC now. Return 0 and let DRM choose some same defaults.  These 3 low-res modes act as a safe base line. Reporting high-res BMC modes might accidentally fry someone's monitor. > > Of course, this implementation has its drawbacks; it doesn't allow users to provide a custom resolution list via debugfs > using the override method when `count == 0`. But this method isn't very important for how our product is used. The upstream Linux kernel is not a product, but a shared commodity. It needs to work in everyone's best interest. Best regards Thomas > > Thanks, > Yongbang. > >> Best regards >> Thomas >> >>>>>    +    drm_edid_connector_update(connector, NULL); >>>>>        count = drm_add_modes_noedid(connector, >>>>>                         connector->dev->mode_config.max_width, >>>>>                         connector->dev->mode_config.max_height); >>>>>        drm_set_preferred_mode(connector, 1024, 768); >>>>>    -out: >>>>> -    drm_edid_free(drm_edid); >>>>> - >>>>>        return count; >>>>>    } >>>>>    @@ -57,10 +50,32 @@ static void hibmc_connector_destroy(struct drm_connector *connector) >>>>>        drm_connector_cleanup(connector); >>>>>    } >>>>>    +static int hibmc_vdac_detect(struct drm_connector *connector, >>>>> +                 struct drm_modeset_acquire_ctx *ctx, >>>>> +                 bool force) >>>>> +{ >>>>> +    struct hibmc_drm_private *priv = to_hibmc_drm_private(connector->dev); >>>>> +    int state = drm_connector_helper_detect_from_ddc(connector, ctx, >>>>> +                             force); >>>> 'state' -> 'status' >>>> >>> Okay. >>> >>>>> +    struct hibmc_vdac *vdac = to_hibmc_vdac(connector); >>>>> + >>>>> +    if (priv->dp.phys_state == connector_status_connected) >>>>> +        return vdac->phys_state = state; >>>> Please only one statement per line. First assign, then return. >>>> >>> Yes, Sorry about that. According to the Linux kernel coding guidelines, this line of code should be split. >>> >>> >>> Thanks, >>> Yongbang. >>> >>>>> + >>>>> +    if (state != vdac->phys_state) >>>>> +        ++connector->epoch_counter; >>>>> +    vdac->phys_state = state; >>>>> + >>>>> +    /* If both the DP and VDAC physical states are disconnected, >>>>> +     * the "connected" status is returned to support KVM display. >>>>> +     */ >>>>> +    return connector_status_connected; >>>> I haven't tried, but I think this should also resolve the problems on my test systems. Great, thanks a lot! I might just >>>> get default resolutions for now, but that's OK. >>>> >>>> Best regards >>>> Thomas >>>> >>>>> +} >>>>> + >>>>>    static const struct drm_connector_helper_funcs >>>>>        hibmc_connector_helper_funcs = { >>>>>        .get_modes = hibmc_connector_get_modes, >>>>> -    .detect_ctx = drm_connector_helper_detect_from_ddc, >>>>> +    .detect_ctx = hibmc_vdac_detect, >>>>>    }; >>>>>      static const struct drm_connector_funcs hibmc_connector_funcs = { >>>>> @@ -130,6 +145,8 @@ int hibmc_vdac_init(struct hibmc_drm_private *priv) >>>>>          connector->polled = DRM_CONNECTOR_POLL_CONNECT | DRM_CONNECTOR_POLL_DISCONNECT; >>>>>    +    vdac->phys_state = connector_status_connected; >>>>> + >>>>>        return 0; >>>>>      err: -- -- Thomas Zimmermann Graphics Driver Developer SUSE Software Solutions Germany GmbH Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)