From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomi Valkeinen Subject: Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts Date: Tue, 28 May 2019 18:53:11 +0300 Message-ID: <7ada0752-6f65-2906-cb29-a47c9490fd57@ti.com> References: <20190510194229.20628-1-aford173@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Adam Ford Cc: Linux-OMAP , Adam Ford , David Airlie , Daniel Vetter , Rob Herring , Mark Rutland , =?UTF-8?Q?Beno=c3=aet_Cousson?= , Tony Lindgren , dri-devel@lists.freedesktop.org, devicetree , Linux Kernel Mailing List List-Id: devicetree@vger.kernel.org Hi, On 28/05/2019 18:09, Adam Ford wrote: > On Tue, May 28, 2019 at 4:11 AM Tomi Valkeinen wrote: >> >> Hi, >> >> On 10/05/2019 22:42, Adam Ford wrote: >>> Currently the source code is compiled using hard-coded values >>> from CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK. This patch allows this >>> clock divider value to be moved to the device tree and be changed >>> without having to recompile the kernel. >>> >>> Signed-off-by: Adam Ford >> >> I understand why you want to do this, but I'm not sure it's a good idea. >> It's really something the driver should figure out, and if we add it to >> the DT, it effectively becomes an ABI. >> >> That said... I'm not sure how good of a job the driver could ever do, as >> it can't know the future scaling needs of the userspace at the time it >> is configuring the clock. And so, I'm not nacking this patch, but I >> don't feel very good about this patch... >> >> The setting also affects all outputs (exluding venc), which may not be >> what the user wants. Then again, I think this setting is really only >> needed on OMAP2 & 3, which have only a single output. But that's the >> same with the current kconfig option, of course. >> >> So, the current CONFIG_OMAP2_DSS_MIN_FCK_PER_PCK is an ugly hack, in my >> opinion, and moving it to DT makes it a worse hack =). But I don't have >> any good suggestions either. > > As it stands the Logic PD OMAP35 and AM37/DM37 boards (SOM-LV and > Torpedo) require this to be hard coded to 4 or it hangs during start. > This is the case for all versions 4.2+. I haven't tested it with > older stuff. Tony has a DM3730 Torpedo kit and reported the hanging > issue to me. I told him to set that value to 4 to make it not hang. > He asked that I move it to the DT to avoid custom kernels. I agree > it's a hack, but if it's create a customized defconfig file for 4 > boards or modify the device tree, it seems like the device tree > approach is less intrusive. Ok, well, I think that's a separate thing from its intended use. The point of the kconfig option is to ensure that the fclk/pclk ratio stays under a certain number to allow enough scaling range. It should never affect a basic non-scaling use case, unless you set it to a too high value, which prevents finding any pclk. Has anyone debugged why the hang is happening? If we can't fix the bug itself, rather than adding a DT option, we could change add a min_fck_per_pck field (as you do), keep the kconfig option, and set the min_fck_per_pck based on the kconfig option, _or_ in case of those affected SoCs, set the min_fck_per_pck even without the kconfig option. Tomi -- Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki