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[209.85.219.177]) by smtp.gmail.com with ESMTPSA id m124sm5872267qkc.70.2021.05.07.14.32.19 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 07 May 2021 14:32:19 -0700 (PDT) Received: by mail-yb1-f177.google.com with SMTP id v188so13728097ybe.1 for ; Fri, 07 May 2021 14:32:19 -0700 (PDT) X-Received: by 2002:a25:d70e:: with SMTP id o14mr15904531ybg.79.1620423139099; Fri, 07 May 2021 14:32:19 -0700 (PDT) MIME-Version: 1.0 References: <20201102181144.3469197-1-swboyd@chromium.org> <20201102181144.3469197-4-swboyd@chromium.org> <161646947526.2972785.6883720652669260316@swboyd.mtv.corp.google.com> In-Reply-To: From: Doug Anderson Date: Fri, 7 May 2021 14:32:07 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3 3/4] drm/bridge: ti-sn65dsi86: Read EDID blob over DDC To: Laurent Pinchart Cc: Stephen Boyd , Andrzej Hajda , Neil Armstrong , Sam Ravnborg , Jernej Skrabec , Jonas Karlman , LKML , dri-devel , Sean Paul Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Tue, Mar 23, 2021 at 12:53 PM Laurent Pinchart wrote: > > Hi Doug, > > On Tue, Mar 23, 2021 at 12:07:27PM -0700, Doug Anderson wrote: > > On Mon, Mar 22, 2021 at 8:17 PM Stephen Boyd wrote: > > > > > > Quoting Laurent Pinchart (2021-03-17 17:20:43) > > > > Hi Stephen, > > > > > > > > Reviving a bit of an old thread, for a question. > > > > > > > > On Mon, Nov 02, 2020 at 10:11:43AM -0800, Stephen Boyd wrote: > > > > > @@ -265,6 +267,23 @@ connector_to_ti_sn_bridge(struct drm_connector *connector) > > > > > static int ti_sn_bridge_connector_get_modes(struct drm_connector *connector) > > > > > { > > > > > struct ti_sn_bridge *pdata = connector_to_ti_sn_bridge(connector); > > > > > + struct edid *edid = pdata->edid; > > > > > + int num, ret; > > > > > + > > > > > + if (!edid) { > > > > > + pm_runtime_get_sync(pdata->dev); > > > > > + edid = pdata->edid = drm_get_edid(connector, &pdata->aux.ddc); > > > > > + pm_runtime_put(pdata->dev); > > > > > > > > Is there any specific reason to use the indirect access method, compared > > > > to the direct method that translates access to an I2C ancillary address > > > > to an I2C-over-AUX transaction (see page 20 of SLLSEH2B) ? The direct > > > > method seems it would be more efficient. > > > > > > No I don't think it matters. I was just using the existing support code > > > that Sean wrote instead of digging into the details. Maybe Sean ran into > > > something earlier and abandoned that approach? > > > > From reading the docs, it sounds as if there _could_ be a reason to > > use the indirect method. Specifically if the i2c host that the bridge > > is on doesn't support clock stretching then the direct method wouldn't > > work according to the docs. Is that something that we'd have to > > reasonably worry about? > > I'm not sure. I'm going through BSP code that uses the direct method, > and I was wondering if it was just an implementation detail. Once I get > the display working on this board, I'll try to find time to compare the > two methods, to see if there's a significatant performance improvement > from the direct method. If there isn't, I won't bother. To follow-up here: We'd actually been using the "direct" method in the BIOS (coreboot) and just found a problem. We're now switching coreboot to the "indirect" mode. Specifically we found that, at least on one panel, the last byte of the extension block (which should have been a CRC) was coming back as 0 when using the "direct" mode. See: https://review.coreboot.org/c/coreboot/+/52959 In addition I was thinking about how to use "direct" mode (ignoring the above problem) and realized that handling the power sequencing at the right time would be hard. Maybe not a problem for you since your bridge is always powered, but I wouldn't know how to model this in general. Specifically if you want to talk over the i2c bus to the panel you've got to power the bridge but I don't think the bridge gets called in the normal code paths. -Doug