Message-ID: <a67dbe24-f15f-cc52-d2d8-22dca554caae@linaro.org>
Date:   Tue, 26 Apr 2022 21:04:31 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Abhinav Kumar <quic_abhinavk@...cinc.com>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Sankeerth Billakanti <quic_sbillaka@...cinc.com>,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Vinod Koul <vkoul@...nel.org>, Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Stephen Boyd <swboyd@...omium.org>, Sean Paul <sean@...rly.run>
Subject: Re: [Freedreno] [PATCH] drm/msm/dp: move add fail safe mode to
 dp_connector_get_mode()

On 26/04/2022 20:56, Doug Anderson wrote:
> Hi,
> 
> On Tue, Apr 26, 2022 at 10:44 AM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
>>
>> On 26/04/2022 20:11, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Tue, Apr 26, 2022 at 10:01 AM Dmitry Baryshkov
>>> <dmitry.baryshkov@...aro.org> wrote:
>>>>
>>>> On 26/04/2022 18:37, Abhinav Kumar wrote:
>>>>> Hi Doug
>>>>>
>>>>> On 4/26/2022 8:20 AM, Doug Anderson wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On Mon, Apr 25, 2022 at 8:35 PM Abhinav Kumar
>>>>>> <quic_abhinavk@...cinc.com> wrote:
>>>>>>>
>>>>>>> On 4/25/2022 7:18 PM, Doug Anderson wrote:
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> On Mon, Apr 25, 2022 at 6:42 PM Abhinav Kumar
>>>>>>>> <quic_abhinavk@...cinc.com> wrote:
>>>>>>>>>
>>>>>>>>>>> 2) When there was a valid EDID but no 640x480 mode
>>>>>>>>>>>
>>>>>>>>>>> This is the equipment specific case and the one even I was a bit
>>>>>>>>>>> surprised. There is a DP compliance equipment we have in-house
>>>>>>>>>>> and while
>>>>>>>>>>> validation, it was found that in its list of modes , it did not
>>>>>>>>>>> have any
>>>>>>>>>>> modes which chromebook supported ( due to 2 lanes ). But my
>>>>>>>>>>> understanding was that, all sinks should have atleast 640x480 but
>>>>>>>>>>> apparently this one did not have that. So to handle this DP
>>>>>>>>>>> compliance
>>>>>>>>>>> equipment behavior, we had to do this.
>>>>>>>>>>
>>>>>>>>>> That doesn't seem right. If there's a valid EDID and the valid EDID
>>>>>>>>>> doesn't contain 640x480, are you _sure_ you're supposed to be adding
>>>>>>>>>> 640x480? That doesn't sound right to me. I've got a tiny display in
>>>>>>>>>> front of me for testing that only has one mode:
>>>>>>>>>>
>>>>>>>>>>        #0 800x480 65.68 800 840 888 928 480 493 496 525 32000
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> As I had wrote, DRM core kicks in only when the count of modes is 0.
>>>>>>>>> Here what is happening is the count was not 0 but 640x480 was not
>>>>>>>>> present in the EDID. So we had to add it explicitly.
>>>>>>>>>
>>>>>>>>> Your tiny display is a display port display?
>>>>>>>>>
>>>>>>>>> I am referring to only display port monitors. If your tiny display is
>>>>>>>>> DP, it should have had 640x480 in its list of modes.
>>>>>>>>
>>>>>>>> My tiny display is actually a HDMI display hooked up to a HDMI to DP
>>>>>>>> (active) adapter.
>>>>>>>>
>>>>>>>> ...but this is a legal and common thing to have. I suppose possibly my
>>>>>>>> HDMI display is "illegal"?
>>>>>>>>
>>>>>>>> OK, so reading through the spec more carefully, I do see that the DP
>>>>>>>> spec makes numerous mentions of the fact that DP sinks _must_ support
>>>>>>>> 640x480. Even going back to DP 1.4, I see section "5.2.1.2 Video
>>>>>>>> Timing Format" says that we must support 640x480. It seems like that's
>>>>>>>> _intended_ to be used only if the EDID read fails, though or if we
>>>>>>>> somehow have to output video without knowledge of the EDID. It seems
>>>>>>>> hard to believe that there's a great reason to assume a display will
>>>>>>>> support 640x480 if we have more accurate knowledge.
>>>>>>>>
>>>>>>>> In any case, I guess I would still say that adding this mode belongs
>>>>>>>> in the DRM core. The core should notice that it's a DP connection
>>>>>>>> (bridge->type == DRM_MODE_CONNECTOR_DisplayPort) and that 640x480 was
>>>>>>>> left out and it should add it. We should also make sure it's not
>>>>>>>> "preferred" and is last in the list so we never accidentally pick it.
>>>>>>>> If DP truly says that we should always give the user 640x480 then
>>>>>>>> that's true for everyone, not just Qualcomm. We should add it in the
>>>>>>>> core. If, later, someone wants to hide this from the UI it would be
>>>>>>>> much easier if they only needed to modify one place.
>>>>>>>>
>>>>>>>
>>>>>>> So I debugged with kuogee just now using the DP compliance equipment.
>>>>>>> It turns out, the issue is not that 640x480 mode is not present.
>>>>>>>
>>>>>>> The issue is that it is not marked as preferred.
>>>>>>>
>>>>>>> Hence we missed this part during debugging this equipment failure.
>>>>>>>
>>>>>>> We still have to figure out the best way to either mark 640x480 as
>>>>>>> preferred or eliminate other modes during the test-case so that 640x480
>>>>>>> is actually picked by usermode.
>>>>>>>
>>>>>>> Now that being said, the fix still doesn't belong in the framework. It
>>>>>>> has to be in the msm/dp code.
>>>>>>>
>>>>>>> Different vendors handle this failure case differently looks like.
>>>>>>>
>>>>>>> Lets take below snippet from i915 as example.
>>>>>>>
>>>>>>> 3361    if (intel_connector->detect_edid == NULL ||
>>>>>>> 3362        connector->edid_corrupt ||
>>>>>>> 3363        intel_dp->aux.i2c_defer_count > 6) {
>>>>>>> 3364            /* Check EDID read for NACKs, DEFERs and corruption
>>>>>>> 3365             * (DP CTS 1.2 Core r1.1)
>>>>>>> 3366             *    4.2.2.4 : Failed EDID read, I2C_NAK
>>>>>>> 3367             *    4.2.2.5 : Failed EDID read, I2C_DEFER
>>>>>>> 3368             *    4.2.2.6 : EDID corruption detected
>>>>>>> 3369             * Use failsafe mode for all cases
>>>>>>> 3370             */
>>>>>>> 3371            if (intel_dp->aux.i2c_nack_count > 0 ||
>>>>>>> 3372                    intel_dp->aux.i2c_defer_count > 0)
>>>>>>> 3373                    drm_dbg_kms(&i915->drm,
>>>>>>> 3374                                "EDID read had %d NACKs, %d
>>>>>>> DEFERs\n",
>>>>>>> 3375                                intel_dp->aux.i2c_nack_count,
>>>>>>> 3376                                intel_dp->aux.i2c_defer_count);
>>>>>>> 3377            intel_dp->compliance.test_data.edid =
>>>>>>> INTEL_DP_RESOLUTION_FAILSAFE;
>>>>>>
>>>>>
>>>>> The reason I pointed to this code is to give an example of how other
>>>>> drivers handle this test-case.
>>>>>
>>>>> We added this patch for 4.2.2.1 and 4.2.2.6 EDID test cases.
>>>>>
>>>>> The challenge here as found out from our discussion here was to mark a
>>>>> particular mode as preferred so that the Chrome usermode can pick it.
>>>>>
>>>>> Now whats happening with that there was always a possibility of two
>>>>> modes being marked as preferred due to this and so-on.
>>>>>
>>>>> We had a pretty long discussion last night and thought of all possible
>>>>> solutions but all of them look like a hack to us in the driver because
>>>>> we end up breaking other things due to this.
>>>>>
>>>>> So we decided that driver is not the place to handle this test case.
>>>>> Since we do have IGT support for chromebooks, we will handle both these
>>>>> test cases there as other vendors do the same way and it works.
>>>>>
>>>>>
>>>>>> Just because Intel DRM has its own solution for something doesn't mean
>>>>>> everyone else should copy them and implement their own solution. Up
>>>>>> until recently DP AUX backlights were baked into different DRM
>>>>>> drivers. A recent effort was made to pull it out. I think the Intel
>>>>>> DRM code was the "first one" to the party and it wasn't clear how
>>>>>> things should be broken up to share with other drivers, so mostly it
>>>>>> did everything itself, but that's not the long term answer.
>>>>>>
>>>>>> I'm not saying that we need to block your change on a full re-design
>>>>>> or anything, but I'm just saying that:
>>>>>>
>>>>>> * You're trying to implement a generic DP rule, not something specific
>>>>>> to Qualcomm hardware. That implies that, if possible, it shouldn't be
>>>>>> in a Qualcomm driver.
>>>>>>
>>>>>> * It doesn't seem like it would be terrible to handle this in the core.
>>>>>>
>>>>>>
>>>>>>> This marks the fail safe mode and IGT test case reads this to set this
>>>>>>> mode and hence the test passes.
>>>>>>>
>>>>>>> We rely on the chromeOS usermode to output pixel data for this test-case
>>>>>>> and not IGT. We use IGT only for video pattern CTS today but this is a
>>>>>>> different test-case which is failing.
>>>>>>>
>>>>>>> ChromeOS usermode will not pick 640x480 unless we mark it as preferred
>>>>>>> or other modes are eliminated.
>>>>>>>
>>>>>>> So we have to come up with the right way for the usermode to pick
>>>>>>> 640x480.
>>>>>>>
>>>>>>> We will discuss this a bit more and come up with a different change.
>>>>>>
>>>>>> Can you provide the exact EDID from the failing test case? Maybe that
>>>>>> will help shed some light on what's going on. I looked at the original
>>>>>> commit and it just referred to 4.2.2.1, which I assume is "EDID Read
>>>>>> upon HPD Plug Event", but that doesn't give details that seem relevant
>>>>>> to the discussion here.
>>>>>
>>>>> Yes so it is 4.2.2.1 and 4.2.2.6.
>>>>>
>>>>> That alone wont give the full picture.
>>>>>
>>>>> So its a combination of things.
>>>>>
>>>>> While running the test, the test equipment published only one mode.
>>>>> But we could not support that mode because of 2 lanes.
>>>>> Equipment did not add 640x480 to the list of modes.
>>>>> DRM fwk will also not add it because count_modes is not 0 ( there was
>>>>> one mode ).
>>>>> So we ended up making these changes.
>>>>
>>>> I think a proper solution might be to rewrite
>>>> drm_helper_probe_single_connector_modes() in the following way:
>>>> - call get_modes()
>>>> - validate the result
>>>> - prune invalid
>>>>
>>>> - if the number of modes is 0, call drm_add_override_edid_modes()
>>>> - validate the result
>>>> - prune invalid
>>>>
>>>> - if the number of modes is still 0, call drm_add_modes_noedid()
>>>> - validate the result
>>>> - prune invalid
>>>>
>>>> [A separate change might happen here after all the checks: if the number
>>>> of modes is still 0 and if it is a DP, enforce adding 640x480 even w/o
>>>> validation. But generally I feel that this shouldn't be necessary
>>>> because the previous step should have added it.]
>>>>
>>>> This way we can be sure that all modes are validated, but still to do
>>>> our best to add something supported to the list of modes.
>>>
>>> I'm partway through implementing / testing something similar to this.
>>> ;-) My logic is slightly different than yours, though. In the very
>>> least I'm not convinced that we want to add the higher resolution
>>> modes (like 1024x768) if all the modes fail to validate. The DP spec
>>> only claims 640x480 is always supported. The higher resolution modes
>>> are for when the EDID fails to read I think. Similarly I'm not
>>> convinced that we should do pruning before deciding on
>>> drm_add_override_edid_modes().
>>
>>
>> I think pruning before drm_add_override_edid_modes() would allow one to
>> use override if the first read returned some modes which are invalid.
> 
> Yeah, I'm less certain about drm_add_override_edid_modes(), but as per
> documented it's only to be used if the EDID failed to read.
> 
> If someone has an actual use case where they need to add the override
> modes for this specific case then we can, but I think it can be done
> separately once someone has an actual use case.
> 
> 
>> Regarding 1024 vs 640. For the restructure we shouldn't change this. And
>> I'd actually point to the following commit message:
>>
>> commit 9632b41f00cc2fb2846569cc99dbeef78e5c94a0
>> Author: Adam Jackson <ajax@...hat.com>
>> Date:   Mon Nov 23 14:23:07 2009 -0500
>>
>>       drm/modes: Fall back to 1024x768 instead of 800x600
>>
>>       This matches the X server's fallback modes when using RANDR 1.2.
>>
>>       See also: http://bugzilla.redhat.com/538761
>>
>>       Signed-off-by: Adam Jackson <ajax@...hat.com>
>>       Signed-off-by: Dave Airlie <airlied@...hat.com>
>>
>>
>> So I'd say, let's leave 1024 as is and just try them if all other modes
>> are invalid.
> 
> I'm pretty strongly against adding 1024x768 when all modes fail to
> validate. Specifically:
> 
> If the EDID fully fails to read then adding these higher resolution
> modes makes sense. We have no knowledge at all about the display in
> this case and so we can guess that some standard higher resolutions
> might make sense. In the case we're dealing with here, we have very
> specific knowledge about what the display said it could handle and we
> can't support any of them. The DP spec _only_ lists 640x480 as a
> required mode so that's the only one we should add.

Then this should be a DP-specific override happening after all the pruning.

-- 
With best wishes
Dmitry

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