Message-ID: <20250827-practical-crimson-jaguarundi-aa356f@houat>
Date: Wed, 27 Aug 2025 13:25:55 +0200
From: Maxime Ripard <mripard@...nel.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Swamil Jain <s-jain1@...com>, h-shenoy@...com, devarsht@...com, 
	vigneshr@...com, praneeth@...com, u-kumar1@...com, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org, jyri.sarha@....fi, 
	maarten.lankhorst@...ux.intel.com, tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch, 
	aradhya.bhatia@...ux.dev
Subject: Re: [PATCH v5 2/3] drm/tidss: Remove max_pclk_khz from tidss display
 features

On Wed, Aug 27, 2025 at 01:39:06PM +0300, Tomi Valkeinen wrote:
> Hi,
> 
> On 27/08/2025 13:34, Maxime Ripard wrote:
> > On Wed, Aug 27, 2025 at 12:49:37PM +0300, Tomi Valkeinen wrote:
> >> On 27/08/2025 12:27, Maxime Ripard wrote:
> >>> On Wed, Aug 27, 2025 at 11:49:22AM +0300, Tomi Valkeinen wrote:
> >>>> On 19/08/2025 22:21, Swamil Jain wrote:
> >>>>> From: Jayesh Choudhary <j-choudhary@...com>
> >>>>>
> >>>>> TIDSS hardware by itself does not have variable max_pclk for each VP.
> >>>>> The maximum pixel clock is determined by the limiting factor between
> >>>>> the functional clock and the PLL (parent to the VP/pixel clock).
> >>>>
> >>>> Hmm, this is actually not in the driver, is it? We're not limiting the
> >>>> pclk based on the fclk.
> >>>>
> >>>>> The limitation that has been modeled till now comes from the clock
> >>>>> (PLL can only be programmed to a particular max value). Instead of
> >>>>> putting it as a constant field in dispc_features, we can query the
> >>>>> DM to see if requested clock can be set or not and use it in
> >>>>> mode_valid().
> >>>>>
> >>>>> Replace constant "max_pclk_khz" in dispc_features with
> >>>>> max_successful_rate and max_attempted_rate, both of these in
> >>>>> tidss_device structure would be modified in runtime. In mode_valid()
> >>>>> call, check if a best frequency match for mode clock can be found or
> >>>>> not using "clk_round_rate()". Based on that, propagate
> >>>>> max_successful_rate and max_attempted_rate and query DM again only if
> >>>>> the requested mode clock is greater than max_attempted_rate. (As the
> >>>>> preferred display mode is usually the max resolution, driver ends up
> >>>>> checking the highest clock the first time itself which is used in
> >>>>> subsequent checks).
> >>>>>
> >>>>> Since TIDSS display controller provides clock tolerance of 5%, we use
> >>>>> this while checking the max_successful_rate. Also, move up
> >>>>> "dispc_pclk_diff()" before it is called.
> >>>>>
> >>>>> This will make the existing compatibles reusable if DSS features are
> >>>>> same across two SoCs with the only difference being the pixel clock.
> >>>>>
> >>>>> Fixes: 7246e0929945 ("drm/tidss: Add OLDI bridge support")
> >>>>> Reviewed-by: Devarsh Thakkar <devarsht@...com>
> >>>>> Signed-off-by: Jayesh Choudhary <j-choudhary@...com>
> >>>>> Signed-off-by: Swamil Jain <s-jain1@...com>
> >>>>> ---
> >>>>>  drivers/gpu/drm/tidss/tidss_dispc.c | 85 +++++++++++++----------------
> >>>>>  drivers/gpu/drm/tidss/tidss_dispc.h |  1 -
> >>>>>  drivers/gpu/drm/tidss/tidss_drv.h   | 11 +++-
> >>>>>  3 files changed, 47 insertions(+), 50 deletions(-)
> >>>>>
> >>>>> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.c b/drivers/gpu/drm/tidss/tidss_dispc.c
> >>>>> index c0277fa36425..c2c0fe0d4a0f 100644
> >>>>> --- a/drivers/gpu/drm/tidss/tidss_dispc.c
> >>>>> +++ b/drivers/gpu/drm/tidss/tidss_dispc.c
> >>>>> @@ -58,10 +58,6 @@ static const u16 tidss_k2g_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
> >>>>>  const struct dispc_features dispc_k2g_feats = {
> >>>>>  	.min_pclk_khz = 4375,
> >>>>>  
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_DPI] = 150000,
> >>>>> -	},
> >>>>> -
> >>>>>  	/*
> >>>>>  	 * XXX According TRM the RGB input buffer width up to 2560 should
> >>>>>  	 *     work on 3 taps, but in practice it only works up to 1280.
> >>>>> @@ -144,11 +140,6 @@ static const u16 tidss_am65x_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
> >>>>>  };
> >>>>>  
> >>>>>  const struct dispc_features dispc_am65x_feats = {
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_DPI] = 165000,
> >>>>> -		[DISPC_VP_OLDI_AM65X] = 165000,
> >>>>> -	},
> >>>>> -
> >>>>>  	.scaling = {
> >>>>>  		.in_width_max_5tap_rgb = 1280,
> >>>>>  		.in_width_max_3tap_rgb = 2560,
> >>>>> @@ -244,11 +235,6 @@ static const u16 tidss_j721e_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
> >>>>>  };
> >>>>>  
> >>>>>  const struct dispc_features dispc_j721e_feats = {
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_DPI] = 170000,
> >>>>> -		[DISPC_VP_INTERNAL] = 600000,
> >>>>> -	},
> >>>>> -
> >>>>>  	.scaling = {
> >>>>>  		.in_width_max_5tap_rgb = 2048,
> >>>>>  		.in_width_max_3tap_rgb = 4096,
> >>>>> @@ -315,11 +301,6 @@ const struct dispc_features dispc_j721e_feats = {
> >>>>>  };
> >>>>>  
> >>>>>  const struct dispc_features dispc_am625_feats = {
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_DPI] = 165000,
> >>>>> -		[DISPC_VP_INTERNAL] = 170000,
> >>>>> -	},
> >>>>> -
> >>>>>  	.scaling = {
> >>>>>  		.in_width_max_5tap_rgb = 1280,
> >>>>>  		.in_width_max_3tap_rgb = 2560,
> >>>>> @@ -376,15 +357,6 @@ const struct dispc_features dispc_am625_feats = {
> >>>>>  };
> >>>>>  
> >>>>>  const struct dispc_features dispc_am62a7_feats = {
> >>>>> -	/*
> >>>>> -	 * if the code reaches dispc_mode_valid with VP1,
> >>>>> -	 * it should return MODE_BAD.
> >>>>> -	 */
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_TIED_OFF] = 0,
> >>>>> -		[DISPC_VP_DPI] = 165000,
> >>>>> -	},
> >>>>> -
> >>>>>  	.scaling = {
> >>>>>  		.in_width_max_5tap_rgb = 1280,
> >>>>>  		.in_width_max_3tap_rgb = 2560,
> >>>>> @@ -441,10 +413,6 @@ const struct dispc_features dispc_am62a7_feats = {
> >>>>>  };
> >>>>>  
> >>>>>  const struct dispc_features dispc_am62l_feats = {
> >>>>> -	.max_pclk_khz = {
> >>>>> -		[DISPC_VP_DPI] = 165000,
> >>>>> -	},
> >>>>> -
> >>>>>  	.subrev = DISPC_AM62L,
> >>>>>  
> >>>>>  	.common = "common",
> >>>>> @@ -1347,25 +1315,57 @@ static void dispc_vp_set_default_color(struct dispc_device *dispc,
> >>>>>  			DISPC_OVR_DEFAULT_COLOR2, (v >> 32) & 0xffff);
> >>>>>  }
> >>>>>  
> >>>>> +/*
> >>>>> + * Calculate the percentage difference between the requested pixel clock rate
> >>>>> + * and the effective rate resulting from calculating the clock divider value.
> >>>>> + */
> >>>>> +unsigned int dispc_pclk_diff(unsigned long rate, unsigned long real_rate)
> >>>>> +{
> >>>>> +	int r = rate / 100, rr = real_rate / 100;
> >>>>> +
> >>>>> +	return (unsigned int)(abs(((rr - r) * 100) / r));
> >>>>> +}
> >>>>> +
> >>>>> +static int check_pixel_clock(struct dispc_device *dispc,
> >>>>> +			     u32 hw_videoport, unsigned long clock)
> >>>>> +{
> >>>>> +	unsigned long round_clock;
> >>>>> +
> >>>>> +	if (dispc->tidss->is_ext_vp_clk[hw_videoport])
> >>>>> +		return 0;
> >>>>> +
> >>>>> +	if (clock <= dispc->tidss->max_successful_rate[hw_videoport])
> >>>>> +		return 0;
> >>>>> +
> >>>>> +	if (clock < dispc->tidss->max_attempted_rate[hw_videoport])
> >>>>> +		return -EINVAL;
> >>>>> +
> >>>>> +	round_clock = clk_round_rate(dispc->vp_clk[hw_videoport], clock);
> >>>>> +
> >>>>> +	if (dispc_pclk_diff(clock, round_clock) > 5)
> >>>>> +		return -EINVAL;
> >>>>> +
> >>>>> +	dispc->tidss->max_successful_rate[hw_videoport] = round_clock;
> >>>>> +	dispc->tidss->max_attempted_rate[hw_videoport] = clock;
> >>>>
> >>>> I still don't think this logic is sound. This is trying to find the
> >>>> maximum clock rate, and optimize by avoiding the calls to
> >>>> clk_round_rate() if possible. That makes sense.
> >>>>
> >>>> But checking for the 5% tolerance breaks it, in my opinion. If we find
> >>>> out that the PLL can do, say, 100M, but we need pclk of 90M, the current
> >>>> maximum is still the 100M, isn't it?
> >>>
> >>> 5% is pretty large indeed. We've been using .5% in multiple drivers and
> >>> it proved to be pretty ok. I would advise you tu use it too.
> >>
> >> The 5% comes from OMAP DSS, where we had to do pixel clock with a few
> >> dividers and multipliers. The rates were quite coarse, and we ended up
> >> having quite a large tolerance.
> >>
> >> I think with tidss, we always have a PLL we control, so we should always
> >> have very exact clocks. So I'm fine with dropping it to .5%. However,
> >> this patch and series is about removing the a-bit-too-hardcoded VP clk
> >> max rate code in the driver, so I would leave everything else to another
> >> series.
> > 
> > Ack
> > 
> >>> It's not clear to me why avoiding a clk_round_rate() call is something
> >>> worth doing though?
> >>
> >> Hard to say if it's worth doing, someone should make some perf tests.
> >> However, afaik, the calls do go to the firmware, so it involves
> >> inter-processor calls. On OMAP DSS checking the clock rates was slow, as
> >> it involved lots of iterating with dividers and multipliers. Perhaps
> >> it's much faster here.
> > 
> > It's not like it's going to be called a lot anyway. It's called once for
> > each mode when EDID are read (using an I2C bus), and then once per
> > commit that change the mode.
> > 
> > Both operations are super slow already, so I'm pretty sure you wouldn't
> > be able to tell.
> > 
> >>> Even caching the maximum rate you have been able to reach before is
> >>> pretty fragile: if the PLL changes its rate, or if a sibling clock has
> >>> set some limits on what the PLL can do, your maximum isn't relevant
> >>> anymore.
> >>
> >> You're right, although afaik it should not happen with TI's SoCs. We
> >> would be in trouble anyway if that were the case (e.g. someone starts
> >> the camera, and suddenly we can't support 1080p anymore).
> >>
> >>> in other words, what's wrong with simply calling clk_round_rate() and
> >>> checking if it's within a .5% deviation?
> >>
> >> This started with discussions how to replace the hardcoded max VP clock
> >> rate (used to quickly weed out impossible rates), which in reality was
> >> actually PLL max clock rate. We don't know the PLL max rate, and can't
> >> query it, so this approach was taken.
> > 
> > If it's fixed by the platform, you have clk_get_max_rate(), but also,
> 
> We have what, where? I don't see clk_get_max_rate(), and when I looked,
> I didn't see any means to find out the limits of a clock.

Sorry, clk_get_(min|max)_rate is something is sent at some point, but never got merged, and I
recalled it did. Patch is here:
https://lore.kernel.org/linux-clk/20220516132527.328190-4-maxime@cerno.tech/

There's clk_hw_get_rate_range(), but it's only available to providers.

Maxime

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