Re: [PATCH] drm/omap: Migrate minimum FCK/PCK ratio from Kconfig to dts

[Tomi Valkeinen]

Hi,

On 28/05/2019 18:09, Adam Ford wrote:
> On Tue, May 28, 2019 at 4:11 AM Tomi Valkeinen <tomi.valkeinen@xxxxxx> 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 <aford173@xxxxxxxxx>
> >
> > 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

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