linux-kernel - Re: [PATCH] drm/vc4: Fix HDMI mode validation

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Message-ID: <096467b2-e617-4353-ce8c-33312bd553da@i2se.com>
Date:   Thu, 26 Mar 2020 18:52:15 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Eric Anholt <eric@...olt.net>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     maxime@...no.tech, linux-rpi-kernel@...ts.infradead.org,
        f.fainelli@...il.com,
        Dave Stevenson <dave.stevenson@...pberrypi.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/vc4: Fix HDMI mode validation

Am 26.03.20 um 18:36 schrieb Nicolas Saenz Julienne:
> On Thu, 2020-03-26 at 18:19 +0100, Stefan Wahren wrote:
>> Am 26.03.20 um 13:20 schrieb Nicolas Saenz Julienne:
>>> Current mode validation impedes setting up some video modes which should
>>> be supported otherwise. Namely 1920x1200@...z.
>>>
>>> Fix this by lowering the minimum HDMI state machine clock to pixel clock
>>> ratio allowed.
>>>
>>> Fixes: 32e823c63e90 ("drm/vc4: Reject HDMI modes with too high of clocks")
>>> Reported-by: Stefan Wahren <stefan.wahren@...e.com>
>>> Suggested-by: Dave Stevenson <dave.stevenson@...pberrypi.com>
>>> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
>>> ---
>>>  drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++----
>>>  1 file changed, 16 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
>>> index cea18dc15f77..340719238753 100644
>>> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
>>> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
>>> @@ -681,11 +681,23 @@ static enum drm_mode_status
>>>  vc4_hdmi_encoder_mode_valid(struct drm_encoder *crtc,
>>>  			    const struct drm_display_mode *mode)
>>>  {
>>> -	/* HSM clock must be 108% of the pixel clock.  Additionally,
>>> -	 * the AXI clock needs to be at least 25% of pixel clock, but
>>> -	 * HSM ends up being the limiting factor.
>>> +	/*
>>> +	 * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
>>> +	 * be faster than pixel clock, infinitesimally faster, tested in
>>> +	 * simulation. Otherwise, exact value is unimportant for HDMI
>>> +	 * operation." This conflicts with bcm2835's vc4 documentation, which
>>> +	 * states HSM's clock has to be at least 108% of the pixel clock.
>>> +	 *
>>> +	 * Real life tests reveal that vc4's firmware statement holds up, and
>>> +	 * users are able to use pixel clocks closer to HSM's, namely for
>>> +	 * 1920x1200@...z. So it was decided to have leave a 1% margin between
>>> +	 * both clocks. Which, for RPi0-3 implies a maximum pixel clock of
>> s/RPi0-3/bcm2835/ ?
> Well as Dave Stevenson stated on the previous thread[1], it's the firmware that
> sets up the HSM limitation. IIUC technically both HSM and pixel clocks could be
> increased. Hence this being more of a RPi limitation than the chip itself.
>
> "Whilst the firmware would appear to use a fixed HSM clock on Pi0-3, I
> don't anticipate there being any issue with varying it. It looks like
> there was the expectation of it being variable in the past, but
> someone has refactored it and either accidentally or deliberately
> given up on the idea."

Okay thanks for the explanation again

So i'm fine this patch

>
> Regards,
> Nicolas
>
> [1] https://www.spinics.net/lists/arm-kernel/msg794591.html
>