RE: [EXT] Re: [v1] drm/arm/mali-dp: Disable checking for required pixel clock rate

[Wen He]

> -----Original Message-----
> From: liviu.dudau@xxxxxxx [mailto:liviu.dudau@xxxxxxx]
> Sent: 2019å5æ16æ 16:24
> To: Wen He <wen.he_1@xxxxxxx>
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Leo Li
> <leoyang.li@xxxxxxx>
> Subject: Re: [EXT] Re: [v1] drm/arm/mali-dp: Disable checking for required
> pixel clock rate
> 
> Caution: EXT Email
> 
> On Thu, May 16, 2019 at 08:10:21AM +0000, Wen He wrote:
> >
> >
> > > -----Original Message-----
> > > From: liviu.dudau@xxxxxxx [mailto:liviu.dudau@xxxxxxx]
> > > Sent: 2019å5æ15æ 23:46
> > > To: Wen He <wen.he_1@xxxxxxx>
> > > Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > Leo Li <leoyang.li@xxxxxxx>
> > > Subject: [EXT] Re: [v1] drm/arm/mali-dp: Disable checking for
> > > required pixel clock rate
> > >
> > >
> > > Hi Wen,
> >
> > Hi Liviu,
> >

Hi Liviu,

> > >
> > > On Wed, May 15, 2019 at 02:42:08AM +0000, Wen He wrote:
> > > > Disable checking for required pixel clock rate if ARCH_LAYERSCPAE
> > > > is enable.
> > > >
> > > > Signed-off-by: Alison Wang <alison.wang@xxxxxxx>
> > > > Signed-off-by: Wen He <wen.he_1@xxxxxxx>
> > > > ---
> > > > change in description:
> > > >       - This check that only supported one pixel clock required clock
> rate
> > > >       compare with dts node value. but we have supports 4 pixel clock
> > > >       for ls1028a board.
> > >
> > > So, your DT says your pixel clock provider is a fixed clock? If you
> > > support more than one rate, you should instead use a real provider
> > > for it. How do you support the 4 pixel clocks?
> > >
> >
> > Yes , the DT node only can provided one pixel clock by using a fixed clock.
> > But we Display Port controller support 4 or more resolutions, each of
> > which requires a set of pixel clocks to drive, and we hope they can
> > switch any resolution we want by some program every times.
> 
> That program can't be some userspace application, because it will have to
> make changes to the hardware and the kernel will not know that things have
> changed under its feet. That leaves the option of the bootloader or some other
> kernel module doing the changes.
> 
> If you have another kernel module that knows how to change clocks, that
> should be implemented using the common clocks infrastructure, at which time
> you can put it in the DT as the clock provider for the pixelclock.
> 

Hi Liviu,

Yes, I think you are right, and even though we didn't implement clocks prepare
/enable/disable interface, but we can notification hardware to change pixel clock
by determining the required pixel clock in each mode once had modeset event
in DP driver.

But the point is how do we meet the conditions for the clock rate check in here? 

As you know, we LS1028A supports 4 modes, every resolution change will to
determine whether the hardware supports the pixel clock required for the resolution
by calling this function malidp_crtc_mode_valid() . assume if we put fixed-clock in DT
node that will can't meet this checking.

Best Regards,
Wen

> If the bootloader does the changes, then the bootloader should edit the DT
> and set the correct value for the pixel clock. Regardless, with your change and
> on your platform the user can request any resolution and the driver will
> silently fail to set that resolution.
> 
> One other problem is the one Robin raised, where the kernel is compiled for
> multiple platforms, like what various Linux distributions do. That kernel will
> either work on other SoC or not, depending on what
> CONFIG_ARCH_LAYERSCAPE is set to.
> 
> In summary, for this patch, it's a NAK. There are proper ways of achieving what
> you need, but this patch is not.
> 
> Best regards,
> Liviu
> 
> >
> > For example, if we set that fixed pixel clock is 27000000 (27Mhz), but
> > user hope can see a group 1080p resolution penguins during startup ,
> > and hope playing a 4k video once system boot up done.
> > Btw, In our board, the 1080p resolution is driven by a 148.5Mhz pixel
> > clock, 4k is driven by a 594Mhz. 27Mhz only can drive 480p resolution.
> >
> > To meet the above user requirements, I was to setup following steps,
> > 1. Add the "video=1920x1080-32@60" to bootargs command line [specify
> > penguins size] 2. Play a 4K video with 4k resolution when system boot up
> done.
> >
> > > Also, not sure what the paragraph above is meant to be. Should it be
> > > part of the commit message?
> > >
> >
> > These comments just want to let you know.
> >
> > > Best regards,
> > > Liviu
> > >
> > >
> > > >  drivers/gpu/drm/arm/malidp_crtc.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > >
> > > > diff --git a/drivers/gpu/drm/arm/malidp_crtc.c
> > > > b/drivers/gpu/drm/arm/malidp_crtc.c
> > > > index 56aad288666e..bb79223d9981 100644
> > > > --- a/drivers/gpu/drm/arm/malidp_crtc.c
> > > > +++ b/drivers/gpu/drm/arm/malidp_crtc.c
> > > > @@ -36,11 +36,13 @@ static enum drm_mode_status
> > > > malidp_crtc_mode_valid(struct drm_crtc *crtc,
> > > >
> >
> > According to our pixel configuration above, Now the variable req_rate
> > value is 148500000 or 59400000, another variable rate value is
> > 27000000, so we will get a warning and display will cannot works well.
> >
> > We're not sure which resolution are user want, and we also can't just
> > offered one resolution to user. so I remove this check on our board, maybe
> it's not good change.
> >
> > I want to know do you have other good suggestion? Thanks.
> >
> > Best Regards,
> > Wen
> >
> > > >       if (req_rate) {
> > > >               rate = clk_round_rate(hwdev->pxlclk, req_rate);
> > > > +#ifndef CONFIG_ARCH_LAYERSCAPE
> > > >               if (rate != req_rate) {
> > > >                       DRM_DEBUG_DRIVER("pxlclk doesn't
> support %ld
> > > Hz\n",
> > > >                                        req_rate);
> > > >                       return MODE_NOCLOCK;
> > > >               }
> > > > +#endif
> > > >       }
> > > >
> > > >       return MODE_OK;
> > > > --
> > > > 2.17.1
> > > >
> > >
> > > --
> > > ====================
> > > | I would like to |
> > > | fix the world,  |
> > > | but they're not |
> > > | giving me the   |
> > >  \ source code!  /
> > >   ---------------
> > >     Â\_(ã)_/Â
> 
> --
> ====================
> | I would like to |
> | fix the world,  |
> | but they're not |
> | giving me the   |
>  \ source code!  /
>   ---------------
>     Â\_(ã)_/Â