[PATCH 2/2] drm/bridge: lt9611: Do not generate HFP/HBP/HSA and EOT packet

Fri Jul 14 06:11:36 UTC 2023

On Thu, 13 Jul 2023 at 23:58, Marek Vasut <marex at denx.de> wrote:
>
> On 7/13/23 20:09, Abhinav Kumar wrote:
> >
> >
> > On 7/12/2023 10:41 AM, Marek Vasut wrote:
> >> On 7/9/23 03:03, Abhinav Kumar wrote:
> >>>
> >>>
> >>> On 7/7/2023 1:47 AM, Neil Armstrong wrote:
> >>>> On 07/07/2023 09:18, Neil Armstrong wrote:
> >>>>> Hi,
> >>>>>
> >>>>> On 06/07/2023 11:20, Amit Pundir wrote:
> >>>>>> On Wed, 5 Jul 2023 at 11:09, Dmitry Baryshkov
> >>>>>> <dmitry.baryshkov at linaro.org> wrote:
> >>>>>>>
> >>>>>>> [Adding freedreno@ to cc list]
> >>>>>>>
> >>>>>>> On Wed, 5 Jul 2023 at 08:31, Jagan Teki
> >>>>>>> <jagan at amarulasolutions.com> wrote:
> >>>>>>>>
> >>>>>>>> Hi Amit,
> >>>>>>>>
> >>>>>>>> On Wed, Jul 5, 2023 at 10:15 AM Amit Pundir
> >>>>>>>> <amit.pundir at linaro.org> wrote:
> >>>>>>>>>
> >>>>>>>>> Hi Marek,
> >>>>>>>>>
> >>>>>>>>> On Wed, 5 Jul 2023 at 01:48, Marek Vasut <marex at denx.de> wrote:
> >>>>>>>>>>
> >>>>>>>>>> Do not generate the HS front and back porch gaps, the HSA gap and
> >>>>>>>>>> EOT packet, as these packets are not required. This makes the
> >>>>>>>>>> bridge
> >>>>>>>>>> work with Samsung DSIM on i.MX8MM and i.MX8MP.
> >>>>>>>>>
> >>>>>>>>> This patch broke display on Dragonboard 845c (SDM845) devboard
> >>>>>>>>> running
> >>>>>>>>> AOSP. This is what I see
> >>>>>>>>> https://people.linaro.org/~amit.pundir/db845c-userdebug/v6.5-broken-display/PXL_20230704_150156326.jpg.
> >>>>>>>>> Reverting this patch fixes this regression for me.
> >>>>>>>>
> >>>>>>>> Might be msm dsi host require proper handling on these updated
> >>>>>>>> mode_flags? did they?
> >>>>>>>
> >>>>>>> The msm DSI host supports those flags. Also, I'd like to point out
> >>>>>>> that the patch didn't change the rest of the driver code. So even if
> >>>>>>> drm/msm ignored some of the flags, it should not have caused the
> >>>>>>> issue. Most likely the issue is on the lt9611 side. I's suspect that
> >>>>>>> additional programming is required to make it work with these flags.
> >>>>>>
> >>>>>> I spent some time today on smoke testing these flags (individually
> >>>>>> and
> >>>>>> in limited combination) on DB845c, to narrow down this breakage to
> >>>>>> one
> >>>>>> or more flag(s) triggering it. Here are my observations in limited
> >>>>>> testing done so far.
> >>>>>>
> >>>>>> There is no regression with MIPI_DSI_MODE_NO_EOT_PACKET when enabled
> >>>>>> alone and system boots to UI as usual.
> >>>>>>
> >>>>>> MIPI_DSI_MODE_VIDEO_NO_HFP always trigger the broken display as in
> >>>>>> the
> >>>>>> screenshot[1] shared earlier as well.
> >>>>>>
> >>>>>> Adding either of MIPI_DSI_MODE_VIDEO_NO_HSA and
> >>>>>> MIPI_DSI_MODE_VIDEO_NO_HBP always result in no display, unless paired
> >>>>>> with MIPI_DSI_MODE_VIDEO_NO_HFP and in that case we get the broken
> >>>>>> display as reported.
> >>>>>>
> >>>>>> In short other than MIPI_DSI_MODE_NO_EOT_PACKET flag, all other flags
> >>>>>> added in this commit break the display on DB845c one way or another.
> >>>>>
> >>>>> I think the investigation would be to understand why samsung-dsim
> >>>>> requires
> >>>>> such flags and/or what are the difference in behavior between MSM
> >>>>> DSI and samsung DSIM
> >>>>> for those flags ?
> >>>>>
> >>>>> If someone has access to the lt9611 datasheet, so it requires
> >>>>> HSA/HFP/HBP to be
> >>>>> skipped ? and does MSM DSI and samsung DSIM skip them in the same
> >>>>> way ?
> >>>>
> >>>> I think there's a mismatch, where on one side this flags sets the
> >>>> link in LP-11 while
> >>>> in HSA/HFP/HPB while on the other it completely removes those
> >>>> blanking packets.
> >>>>
> >>>> The name MIPI_DSI_MODE_VIDEO_NO_HBP suggests removal of HPB, not
> >>>> LP-11 while HPB.
> >>>> the registers used in both controllers are different:
> >>>> - samsung-dsim: DSIM_HBP_DISABLE_MODE
> >>>> - msm dsi: DSI_VID_CFG0_HBP_POWER_STOP
> >>>>
> >>>> The first one suggest removing the packet, while the second one
> >>>> suggests powering
> >>>> off the line while in the blanking packet period.
> >>>>
> >>>> @Abhinav, can you comment on that ?
> >>>>
> >>>
> >>> I dont get what it means by completely removes blanking packets in DSIM.
> >>
> >> MIPI_DSI_MODE_VIDEO_NO_HFP means the HBP period is just skipped by DSIM.
> >>
> >> Maybe there is a need for new set of flags which differentiate between
> >> HBP skipped (i.e. NO HBP) and HBP LP11 ?
> >>
> >
> > No, the section of the MIPI DSI spec I posted below clearly states there
> > are two options:
> >
> > 1) send blanking packets during those periods
> > 2) transition to LP11 during those periods
> >
> > There is no 3rd option in the spec of not doing both like what you are
> > suggesting. So DSIM should also be only transitioning to LP11 during
> > those periods if its not sending the blanking packets with those flags set.
> >
> > So, there is no need for any new set of flags to differentiate.
> >
> > The flags and their interpretation is correct in MSM driver. I cannot
> > comment on what exactly DSIM does with those flags.
>
> How do you explain the comment in include/drm/drm_mipi_dsi.h:
>
> 128 /* disable hback-porch area */
> 129 #define MIPI_DSI_MODE_VIDEO_NO_HBP      BIT(6)
>
> Esp. the "disable" part. That to me reads as "don't send HBP packet".
>
> Where do you see that quote above in the DSI spec (which chapter and
> which version do you read) ?
>
> >>> It should be replacing those periods with LP11 too.
> >>>
> >>> The traffic mode being used on this bridge is
> >>> MIPI_DSI_MODE_VIDEO_SYNC_PULSE which is "Non-Burst Mode with Sync
> >>> Pulses".
> >>>
> >>> As per this traffic mode in the DSI spec,
> >>>
> >>> "Normally, periods shown as HSA (Horizontal Sync Active), HBP
> >>> (Horizontal Back Porch) and HFP (Horizontal Front Porch) are filled
> >>> by Blanking Packets, with lengths (including packet overhead)
> >>> calculated to match the period specified by the peripheral’s data
> >>> sheet. Alternatively, if there is sufficient time to transition from
> >>> HS to LP mode and back again, a timed interval in LP mode may
> >>> substitute for a Blanking Packet, thus saving power. During HSA, HBP
> >>> and HFP periods, the bus should stay in the LP-11 state."
> >>>
> >>> So we can either send the blanking packets or transition to LP state
> >>> and those 3 flags are controlling exactly that during those periods
> >>> for MSM driver.
> >>>
> >>> If you stop sending the blanking packets, you need to replace that
> >>> gap with LP.
> >>
> >> I don't think that's what MIPI_DSI_MODE_VIDEO_NO_HBP means, the way I
> >> understand MIPI_DSI_MODE_VIDEO_NO_HBP is that it skips the HBP
> >> completely. So if you want HBP, then do not set
> >> MIPI_DSI_MODE_VIDEO_NO_HBP . And if you want LP11 during HBP, that is
> >> I think up to the controller (or maybe another new flag?).
> >>
> >
> > No, there is no need of another new flag. There are only two options as
> > per the spec.
> >
> > In fact, as per my checking with more folks, requiring LP11 during those
> > periods is something very rare.
> >
> > Because usually horizontal period is usually a very short period, most
> > of the time we do not use the LP11 option and send the blanking packets
> > instead.
> >
> > So its something very unusual for DSIM.
> >
> > That being said, I still think my previous question is important.
> >
> > 1) What is the difference between the resolution you are trying Vs what
> > Amit is trying?
> >
> > 2) Are you both using just standard HDMI monitors?
>
> What is a "standard HDMI monitor" ?
> I use DELL U2713HM .

I see this breakage on portable HDMI monitors Viewsonic TD1655 [1] and
Cocopar Z173FH7F [2], both running at 1920x1080 resolution.

Regards,
Amit Pundir
[1] https://www.amazon.in/ViewSonic-TD1655-Portable-Touchscreen-Frameless/dp/B08778F756
[2] https://www.amazon.com/Portable-Monitor-FreeSync-Speaker-Nintendo/dp/B07ZLXCVPN

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