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Tue, 6 Oct 2026 03:49:26 -0500 Received: from [172.24.19.94] (ltpw0ewr2k.dhcp.ti.com [172.24.19.94]) by lelvem-mr06.itg.ti.com (8.18.1/8.18.1) with ESMTP id 6968nKNf1927510; Tue, 6 Oct 2026 03:49:21 -0500 Message-ID: Date: Tue, 6 Oct 2026 14:19:20 +0530 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 v3] drm/bridge: cdns-mhdp8546: Add suspend resume support to the bridge driver To: Tomi Valkeinen CC: , , , , , , , , , , , , , , , , References: <20260601095041.3042950-1-a-kumar2@ti.com> <3dbf5a77-c314-4511-ab63-1b1ecd2e16c1@ideasonboard.com> <1c8dfc23-c03c-4c98-ae52-c66ffea244e6@ti.com> <982ffabf-4709-427a-b2b6-481eb074f2d0@ideasonboard.com> Content-Language: en-US From: "Kumar, Abhash" In-Reply-To: <982ffabf-4709-427a-b2b6-481eb074f2d0@ideasonboard.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN1PEPF00036F41:EE_|PH0PR10MB4406:EE_ X-MS-Office365-Filtering-Correlation-Id: 78fbe623-d687-4639-24cf-08df2386ce9e X-LD-Processed: e5b49634-450b-4709-8abb-1e2b19b982b7,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|36860700016|376014|82310400026|7416014|23010399003|4143699003|56012099006|10067099003|22082099003|18002099003|6133799003; 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Firmware >>>> is loaded only when resuming from a successful suspend-resume cycle. >>> >>> It's not clear from the patch if this is a fix or improvement. It >>> sounds a bit like a fix, but it doesn't mention any kind of issue in >>> the driver. So, why is this patch needed? >> >> The driver lacked support for suspend-resume as stated in the todo on >> the driver, So the patch adds this improvement. > > The patch doesn't remove any todo lines. Was that just a miss, or is > there more to add wrt. PM? Yeah, i will remove the TODO in the next revision. > > What does it mean it didn't support PM? Does the driver not work after > suspend-resume cycle? Or does the driver prevent a proper suspend? When we resume, the cdns_mhdp_link_up function fails. Failure log at resume: [  55.329452] tidss 4a00000.dss: PM: calling tidss_resume [tidss] @ 1053, parent: bus@100000 [ 183.381228] cdns-mhdp8546 a000000.bridge: Failed to read receiver capabilities [ 191.391548] cdns-mhdp8546 a000000.bridge: get block[0] edid failed: -110 [ 193.392713] cdns-mhdp8546 a000000.bridge: Failed to read register [ 261.415544] cdns-mhdp8546 a000000.bridge: Failed to read register [ 263.946153] tidss 4a00000.dss: Timeout waiting for framedone on crtc 0 [ 391.991358] cdns-mhdp8546 a000000.bridge: Failed to read receiver capabilities [ 392.026847] tidss 4a00000.dss: PM: tidss_resume [tidss] returned 0 after 336689133 usecs I guess it is fair to say that the driver does not work after suspend-resume cycle. >>>> If resuming due to an aborted suspend, loading the firmware is not >>>> possible because the uCPU's IMEM is only accessible after a reset >>>> and the >>>> bridge has not gone through a reset in this case. Hence, Activate the >>>> firmware that is already loaded. >>>> >>>> Use genpd_notifier to get the power domain status of the bridge and >>>> accordingly load the firmware. >>>> >>>> Additionally, introduce phy_power_off/on to control the power to >>>> the phy. >>> >>> If you write "also" or "additionally" or such in a commit desc, you >>> should stop and think if that part should actually be a separate >>> patch. Also, why is that change needed? >> >> The phy device could be powered off while resuming. So we are >> explicitly powering it on. >> >> The phy driver api also recommends to always call phy_init() first >> followed by a phy_power_on(). >> >> "Some PHY drivers may not implement `phy_init` or `phy_power_on`, but >> controllers should always call these functions to be compatible with >> other PHYs" > > It still sounds like a separate patch to me: the current driver is > missing phy_power_on/off from the probe/remove functions. > I will add that as a separate patch >>> Overall, this sounds fragile/hacky to me. >>> >>> The first thing is that usually you shouldn't use system suspend/ >>> resume in a bridge driver. When a system suspend happend, the >>> display pipeline will be disabled, so this driver will get an >>> atomic_disable() call, and enable when resuming. You can use runtime >>> PM hooks if you need resume/suspend hooks. >>> >> Thanks for the suggestion, I will use the runtime PM instead. >> >>> The second thing is the PD notifier. Is there really no way we can >>> see the state from the MDHP IP registers? >> >> The other way that i found was to read the MHDP KEEP_ALIVE_p register >> twice to know if the firmware is incrementing the counter. >> >> Based on that we can decide if the bridge is active or not. Do you >> think this approach would be okay over the PD notifier? > I think it would be best to be able somehow to ask this from the HW to > find the true state, instead of guessing it second hand from the PD > notifier (which also doesn't tell us the initial HW state at probe). > > KEEP_ALIVE_p sounds fine. Or what does the mdhp IP do if you send a > message to the firmware when it's not up? Say, if you always do > cdns_mhdp_set_firmware_active, what happens if the FW has not been > loaded? I would guess that there's a timeout, and that could be used > to find out the FW is not up. > Yes, there is a 2 second timeout if we send a message and the firmware is not up. I would prefer the KEEP_ALIVE_p method, as that is a better indication of the firmware being active. > Also, if the IMEM is not accessible and you load the FW, what happens? If IMEM is not accessible and we load the FW, the kernel crashes due to S-error. Thanks, Abhash