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

[Biju Das]

Hi Adam and all,

> Subject: Re: imx8mm lcdif->dsi->adv7535 no video, no errors
> 
> On Thu, Aug 4, 2022 at 9:52 AM Dave Stevenson
> <dave.stevenson at raspberrypi.com> wrote:
> >
> > On Thu, 4 Aug 2022 at 13:51, Marco Felsch <m.felsch at pengutronix.de>
> wrote:
> > >
> > > 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.
> >
> > I'd missed that bit in the driver where it appears to drop to 3 lanes
> > for pixel clock < 80000 via a mipi_dsi_detach and _attach. Quirky, but
> > probably the only way it can be achieved in the current framework.
> >
> > > 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.
> >
> > If you change your link frequency, it may be worth trying a lower
> > resolution again such as 720x480 @ 60fps on 2 lanes. (720480 at 60 on 4
> > lanes is again listed as mandatory for using the timing generator).
> >
> > > > 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?
> >
> > Sorry, running without the use of the timing generator is the issue.
> > It is mandatory in some modes, but supported in all modes. Always
> > using it should therefore avoid not using it in one of the mandatory
> > modes (the list looks a little arbitrary).
> >
> > > > 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?
> >
> > BCM2711 which supported dual HDMI natively.
> > Our investigation of ADV7535 was when trying to build what became
> > Pi400 using BCM2710/BCM2837 (only has a single HDMI output). Whilst I
> > do have the prototype, the ADV was wired up weirdly with I2C so I
> > never really got it running with Linux.
> 
> I think I have convinced myself that the DSIM is working good enough to
> match that of the NXP.
> 
> I've gone through and made a list of the register differences between a
> working display using NXP's kernel and the non-working display.  I've
> identified a small handful of registers on both the CEC bank of
> registers and main set of registers.
> 
> I noticed that the working NXP version doesn't rescale the number of
> lanes based on the clock rate, and it stays fixed at 4 lanes.

Does it mean theoretically rescale of lanes is not required??
At least 2 platforms can work with fixed 4 lanes at 720p.

and looks like few platforms have display stability issue working with 4 lanes at 720p, 
so, as a workaround they changed to 3 lanes based on clock rate to make it work.

Can you please confirm, is my understanding correct?

Note:
 On Renesas RZ/G2L platform, 720p with 3 lanes will work, but it needs
 different pll parameters to generate the dot clock to work.
 
Cheers,
Biju