Re: [PATCH] drm/msm/a6xx: fix for kernels without CONFIG_NVMEM

[Jonathan Marek]

On 2/17/21 3:18 PM, Rob Clark wrote:
> On Wed, Feb 17, 2021 at 11:08 AM Jordan Crouse <jcrouse@xxxxxxxxxxxxxx> wrote:
> > On Wed, Feb 17, 2021 at 07:14:16PM +0530, Akhil P Oommen wrote:
> > > On 2/17/2021 8:36 AM, Rob Clark wrote:
> > > > On Tue, Feb 16, 2021 at 12:10 PM Jonathan Marek <jonathan@xxxxxxxx> wrote:
> > > > > Ignore nvmem_cell_get() EOPNOTSUPP error in the same way as a ENOENT error,
> > > > > to fix the case where the kernel was compiled without CONFIG_NVMEM.
> > > > >
> > > > > Fixes: fe7952c629da ("drm/msm: Add speed-bin support to a618 gpu")
> > > > > Signed-off-by: Jonathan Marek <jonathan@xxxxxxxx>
> > > > > ---
> > > > >   drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 6 +++---
> > > > >   1 file changed, 3 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > > index ba8e9d3cf0fe..7fe5d97606aa 100644
> > > > > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > > @@ -1356,10 +1356,10 @@ static int a6xx_set_supported_hw(struct device *dev, struct a6xx_gpu *a6xx_gpu,
> > > > >
> > > > >          cell = nvmem_cell_get(dev, "speed_bin");
> > > > >          /*
> > > > > -        * -ENOENT means that the platform doesn't support speedbin which is
> > > > > -        * fine
> > > > > +        * -ENOENT means no speed bin in device tree,
> > > > > +        * -EOPNOTSUPP means kernel was built without CONFIG_NVMEM
> > > >
> > > > very minor nit, it would be nice to at least preserve the gist of the
> > > > "which is fine" (ie. some variation of "this is an optional thing and
> > > > things won't catch fire without it" ;-))
> > > >
> > > > (which is, I believe, is true, hopefully Akhil could confirm.. if not
> > > > we should have a harder dependency on CONFIG_NVMEM..)
> > > IIRC, if the gpu opp table in the DT uses the 'opp-supported-hw' property,
> > > we will see some error during boot up if we don't call
> > > dev_pm_opp_set_supported_hw(). So calling "nvmem_cell_get(dev, "speed_bin")"
> > > is a way to test this.
> > >
> > > If there is no other harm, we can put a hard dependency on CONFIG_NVMEM.
> >
> > I'm not sure if we want to go this far given the squishiness about module
> > dependencies. As far as I know we are the only driver that uses this seriously
> > on QCOM SoCs and this is only needed for certain targets. I don't know if we
> > want to force every target to build NVMEM and QFPROM on our behalf. But maybe
> > I'm just saying that because Kconfig dependencies tend to break my brain (and
> > then Arnd has to send a patch to fix it).
>
> Hmm, good point.. looks like CONFIG_NVMEM itself doesn't have any
> other dependencies, so I suppose it wouldn't be the end of the world
> to select that.. but I guess we don't want to require QFPROM
>
> I guess at the end of the day, what is the failure mode if you have a
> speed-bin device, but your kernel config misses QFPROM (and possibly
> NVMEM)?  If the result is just not having the highest clk rate(s)
> available, that isn't the end of the world.  But if it makes things
> not-work, that is sub-optimal.  Generally, especially on ARM, kconfig
> seems to be way harder than it should be to build a kernel that works,
> if we could somehow not add to that problem (for both people with a6xx
> and older gens) that would be nice ;-)

There is a "imply" kconfig option which solves exactly this problem. 
(you would "imply NVMEM" instead of "select NVMEM". then it would be 
possible to disable NVMEM but it would get enabled by default)

> BR,
> -R