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[209.132.180.67]) by mx.google.com with ESMTP id y16si13322934pgj.169.2019.07.22.23.17.01; Mon, 22 Jul 2019 23:17:17 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@chromium.org header.s=google header.b=GGeXaV2U; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=chromium.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730443AbfGVUyX (ORCPT + 99 others); Mon, 22 Jul 2019 16:54:23 -0400 Received: from mail-pg1-f195.google.com ([209.85.215.195]:38638 "EHLO mail-pg1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727547AbfGVUyW (ORCPT ); Mon, 22 Jul 2019 16:54:22 -0400 Received: by mail-pg1-f195.google.com with SMTP id f5so9406738pgu.5 for ; Mon, 22 Jul 2019 13:54:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=aHUdEX2ULV+TDjjhsxnkJoyyNmoo7KbS8aN0PdtSzBA=; b=GGeXaV2UIdvQLIYZetuz+R47tXFQa2bSV4X/3q7FX28x5V7fMw1Zf0JaF+D3OcoujM VoZ8HXGilcEUvYhpUDnJWW6U7SNJ4bf6O6iuULjnhWMB0kJx4c5spqUUgPN5D5q3czIz 1Z/WCCz0fXtchf4xoANXGVOI+rcG1Jz0phIiE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=aHUdEX2ULV+TDjjhsxnkJoyyNmoo7KbS8aN0PdtSzBA=; b=igocKC47Xoww7J5a4ZFXxs48oXsBt0jM+KX4QQbYEjTtZrpxqQ2BQ1xnkFPDsKt/09 8fxptQYALKqBd+JMzHwBySIa4wZTYRphtz1e8OLxyCqyp5BOQJZLq2d4ZsD8XGdzypvP Rn7htE/URk6JHH7JXjatfH3g9MuCl3dA72cvQj0/ipFbX44kixm0tmXdKFBCZMOXpCqu EOe9FMGnfwAgQ0WaXqzQXNUNYAOiFtsmlRZLyqMPYBDuGtnKJY6GetgQqkprBncCkIMy 6FAGB9S9qdNDTUoArRA1URSspEesLgo7nWk5j4opLuz0evEjq+Hm/d49Osqv+eT5kdgA 2lYA== X-Gm-Message-State: APjAAAUGyz8GJCNBru9H32Y4ODhx3XQj5oxogprCsgLSUneTCJK9B/9o tCOI8rsi0Dhi2jCp3hfh1kp6zg== X-Received: by 2002:a65:5c02:: with SMTP id u2mr3592167pgr.367.1563828861466; Mon, 22 Jul 2019 13:54:21 -0700 (PDT) Received: from localhost ([2620:15c:202:1:75a:3f6e:21d:9374]) by smtp.gmail.com with ESMTPSA id c8sm45616120pjq.2.2019.07.22.13.54.20 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 22 Jul 2019 13:54:20 -0700 (PDT) Date: Mon, 22 Jul 2019 13:54:19 -0700 From: Matthias Kaehlcke To: Doug Anderson Cc: Andrzej Hajda , Laurent Pinchart , David Airlie , Daniel Vetter , dri-devel , LKML , Jose Abreu , Neil Armstrong , Adam Jackson Subject: Re: [PATCH v2] drm/bridge: dw-hdmi: Refuse DDC/CI transfers on the internal I2C controller Message-ID: <20190722205419.GY250418@google.com> References: <20190722181945.244395-1-mka@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 22, 2019 at 01:12:40PM -0700, Doug Anderson wrote: > Hi, > > On Mon, Jul 22, 2019 at 11:19 AM Matthias Kaehlcke wrote: > > > > The DDC/CI protocol involves sending a multi-byte request to the > > display via I2C, which is typically followed by a multi-byte > > response. The internal I2C controller only allows single byte > > reads/writes or reads of 8 sequential bytes, hence DDC/CI is not > > supported when the internal I2C controller is used. The I2C > > transfers complete without errors, however the data in the response > > is garbage. Abort transfers to/from slave address 0x37 (DDC) with > > -EOPNOTSUPP, to make it evident that the communication is failing. > > > > Signed-off-by: Matthias Kaehlcke > > --- > > Changes in v2: > > - changed DDC_I2C_ADDR to DDC_CI_ADDR > > --- > > drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c > > index 045b1b13fd0e..28933629f3c7 100644 > > --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c > > +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c > > @@ -35,6 +35,7 @@ > > > > #include > > > > +#define DDC_CI_ADDR 0x37 > > #define DDC_SEGMENT_ADDR 0x30 > > > > #define HDMI_EDID_LEN 512 > > @@ -322,6 +323,13 @@ static int dw_hdmi_i2c_xfer(struct i2c_adapter *adap, > > u8 addr = msgs[0].addr; > > int i, ret = 0; > > > > + if (addr == DDC_CI_ADDR) > > + /* > > + * The internal I2C controller does not support the multi-byte > > + * read and write operations needed for DDC/CI. > > + */ > > + return -EOPNOTSUPP; > > + > > In theory we could also solve this by detecting (in other parts of the > function) the bad multi-byte read/write operations. That would maybe > be more generic (AKA it would more properly handle random userspace > tools fiddling with i2c-dev) but add complexity to the code. But how do you know it's an unsupported operation, and not the driver knowing the wacky limitations doing something valid. E.g. you get the sequence: 0x01 0x0a 0x0b 0x0c 0x0d This could be interpreted as "send the above bytes to the slave" or as "send 0x0a to address 0x01, 0x0b to 0x02, 0x0c to 0x03 and 0x0d to 0x04" (at least by this driver ;-) . The latter could be intended. > ...possibly a better long-term solution is to just totally stop > emulating a regular i2c adapter when the hardware just doesn't support > that. In theory we could remove the "drm_get_edid()" call in > dw_hdmi_connector_get_modes() and replace it with a direct call to > drm_do_get_edid() if we're using the built-in adapter. Possibly > "tda998x_drv.c" would be a good reference. If we did that, we could > remove all the weird / hacky i2c code in this driver. yes, that would be another and probably better option than trying to detect unsupported transaction. > Since the bigger cleanup seems like a bit much to ask, I'm OK with > this as a stopgap to at least prevent userspace tools from getting > confused. Thus: > > Reviewed-by: Douglas Anderson Thanks!