imx8mm lcdif->dsi->adv7535 no video, no errors

[Marco Felsch]

Hi Dave,

On 22-08-04, Dave Stevenson wrote:
> Hi Marco
> 
> On Thu, 4 Aug 2022 at 10:38, Marco Felsch <m.felsch at pengutronix.de> wrote:
> >
> > Hi Dave, Adam,
> >
> > On 22-08-03, Dave Stevenson wrote:
> > > Hi Adam
> > >
> > > On Wed, 3 Aug 2022 at 12:03, Adam Ford <aford173 at gmail.com> wrote:
> >
> > ...
> >
> > > > > Did managed to get access to the ADV7535 programming guide? This is the
> > > > > black box here. Let me check if I can provide you a link with our repo
> > > > > so you can test our current DSIM state if you want.
> > > >
> > > > I do have access to the programming guide, but it's under NDA, but
> > > > I'll try to answer questions if I can.
> > >
> > > Not meaning to butt in, but I have datasheets for ADV7533 and 7535
> > > from previously looking at these chips.
> >
> > Thanks for stepping into :)
> >
> > > Mine fairly plainly states:
> > > "The DSI receiver input supports DSI video mode operation only, and
> > > specifically, only supports nonburst mode with sync pulses".
> >
> > I've read this also, and we are working in nonburst mode with sync
> > pulses. I have no access to an MIPI-DSI analyzer therefore I can't
> > verify it.
> >
> > > Non-burst mode meaning that the DSI pixel rate MUST be the same as the
> > > HDMI pixel rate.
> >
> > On DSI side you don't have a pixel-clock instead there is bit-clock.
> 
> You have an effective pixel clock, with a fixed conversion for the
> configuration.
> 
> DSI bit-clock * number of lanes / bits_per_pixel = pixel rate.
> 891Mbit/s * 4 lanes / 24bpp = 148.5 Mpixels/s

Okay, I just checked the bandwidth which must equal.

> As noted elsewhere, the DSI is DDR, so the clock lane itself is only
> running at 891 / 2 = 445.5MHz.
> 
> > > Section 6.1.1 "DSI Input Modes" of adv7533_hardware_user_s_guide is
> > > even more explicit about the requirement of DSI timing matching
> >
> > Is it possible to share the key points of the requirements?
> 
> "Specifically the ADV7533 supports the Non-Burst Mode with syncs. This
> mode requires real time data generation as a pulse packet received
> becomes a pulse generated. Therefore this mode requires a continuous
> stream of data with correct video timing to avoid any visual
> artifacts."
> 
> LP mode is supported on data lanes. Clock lane must remain in HS mode.
> 
> "... the goal is to accurately convey DPI-type timing over DSI. This
> includes matching DPI pixel-transmission rates, and widths of timing
> events."

Thanks for sharing.

> > > The NXP kernel switching down to an hs_clk of 445.5MHz would therefore
> > > be correct for 720p operation.
> >
> > It should be absolute no difference if you work on 891MHz with 2 lanes
> > or on 445.5 MHz with 4 lanes. What must be ensured is that you need the
> > minimum required bandwidth which is roughly: 1280*720*24*60 = 1.327
> > GBps.
> 
> Has someone changed the number of lanes in use? I'd missed that if so,
> but I'll agree that 891MHz over 2 lanes should work for 720p60.

The ADV driver is changing it autom. but this logic is somehow odd and
there was already a approach to stop the driver doing this.

To sync up: we have two problems:
  1) The 720P mode with static DSI host configuration isn't working
     without hacks.
  2) The DSI link frequency should changed as soon as required
     automatically. So we can provide all modes.

I would concentrate on problem 1 first before moving on to the 2nd.

> I have just noted that 720p59.94 at 24bpp on 4 lanes is listed as one
> of the modes that is mandatory to use the timing generator (reg 0x27
> bit 7 = 1). On 2 lanes it is not required.
> I don't know why it's referencing the 1000/1001 pixel clock rates and
> not the base one, as it's only a base clock change with the same
> timing (74.176MHz clock instead of 74.25MHz).

Interesting! I would like to know how the HDMI block gets fetched by the
DSI block and how the timing-generator can influence this in good/bad
way. So that we know what DSI settings (freq, lanes) are sufficient.

> > > If you do program the manual DSI divider register to allow a DSI pixel
> > > rate of 148.5MHz vs HDMI pixel rate of 74.25MHz, you'd be relying on
> >
> > There is no such DSI pixel rate to be precise, we only have a DSI bit
> > clock/rate.
> >
> > > the ADV753x having at least a half-line FIFO between DSI rx and HDMI
> > > tx to compensate for the differing data rates. I see no reference to
> > > such, and I'd be surprised if it was more than a half dozen pixels to
> > > compensate for the jitter in the cases where the internal timing
> > > generator is mandatory due to fractional bytes.
> >
> > This is interesting and would proofs our assumption that the device
> > don't have a FIFO :)
> >
> > Our assumptions (we don't have the datasheet/programming manual):
> >   - HDMI part is fetching 3 bytes per HDMI pixclk
> >   - Ratio between dsi-clk and hdmi-pixelclk must be 3 so the DSI and
> >     HDMI are in sync. So from bandwidth pov there are no differences
> >     between:
> >       - HDMI: 74.25 MHz * 24 Bit  = 1782.0 MBit/s
> >       - DSI:    891 MHz * 2 lanes = 1782.0 MBit/s (dsi-clock: 445.5 )
> >       - DSI:  445.5 MHz * 4 lanes = 1782.0 MBit/s (dsi-clock: 222.75)
> >
> >     But the ratio is different and therefore the faster clocking option
> >     let something 'overflow'.
> 
> I'll agree that all looks consistent.
> 
> > Anyway, but all this means that Adam should configure the
> > burst-clock-rate to 445.5 and set the lanes to 4. But this doesn't work
> > either and now we are back on my initial statement -> the driver needs
> > some attention.
> 
> Things always need attention :-)

^^

> I suspect that it's the use of the timing generator that is the issue.
> The programming guide does recommend using it for all modes, so that
> would be a sensible first step.

But I tested it without the timing-generator too. Can you or Adam verify
the timing-generator diable logic?

> I will say that we had a number of issues getting this chip to do
> anything, and it generally seemed happier on 2 or 3 lanes instead of
> 4. Suffice to say that we abandoned trying to use it, despite some
> assistance from ADI.

Even more interessting, what is your alternative to this chip?

Regards,
  Marco