Re: [PATCH v2] phy: dphy: Correct clk_pre parameter

[Neil Armstrong]

Hi,

On 19/01/2022 11:11, Neil Armstrong wrote:
> Hi,
> 
> On 19/01/2022 11:01, Liu Ying wrote:
>> Hi Neil,
>>
>> On Wed, 2022-01-19 at 10:11 +0100, Neil Armstrong wrote:
>>> On 19/01/2022 09:40, Neil Armstrong wrote:
>>>> Hi,
>>>>
>>>> On 19/01/2022 03:37, Liu Ying wrote:
>>>>> The D-PHY specification (v1.2) explicitly mentions that the T-CLK-PRE
>>>>> parameter's unit is Unit Interval(UI) and the minimum value is 8.  Also,
>>>>> kernel doc of the 'clk_pre' member of struct phy_configure_opts_mipi_dphy
>>>>> mentions that it should be in UI.  However, the dphy core driver wrongly
>>>>> sets 'clk_pre' to 8000, which seems to hint that it's in picoseconds.
>>>>> And, the kernel doc of the 'clk_pre' member wrongly says the minimum value
>>>>> is '8 UI', instead of 8.
>>>>>
>>>>> So, let's fix both the dphy core driver and the kernel doc of the 'clk_pre'
>>>>> member to correctly reflect the T-CLK-PRE parameter's unit and the minimum
>>>>> value according to the D-PHY specification.
>>>>>
>>>>> I'm assuming that all impacted custom drivers shall program values in
>>>>> TxByteClkHS cycles into hardware for the T-CLK-PRE parameter.  The D-PHY
>>>>> specification mentions that the frequency of TxByteClkHS is exactly 1/8
>>>>> the High-Speed(HS) bit rate(each HS bit consumes one UI).  So, relevant
>>>>> custom driver code is changed to program those values as
>>>>> DIV_ROUND_UP(cfg->clk_pre, BITS_PER_BYTE), then.
>>>>>
>>>>> Note that I've only tested the patch with RM67191 DSI panel on i.MX8mq EVK.
>>>>> Help is needed to test with other i.MX8mq, Meson and Rockchip platforms,
>>>>> as I don't have the hardwares.
>>>>>
>>>>> Fixes: 2ed869990e14 ("phy: Add MIPI D-PHY configuration options")
>>>>> Cc: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
>>>>> Cc: Neil Armstrong <narmstrong@xxxxxxxxxxxx>
>>>>> Cc: Robert Foss <robert.foss@xxxxxxxxxx>
>>>>> Cc: Laurent Pinchart <Laurent.pinchart@xxxxxxxxxxxxxxxx>
>>>>> Cc: Jonas Karlman <jonas@xxxxxxxxx>
>>>>> Cc: Jernej Skrabec <jernej.skrabec@xxxxxxxxx>
>>>>> Cc: David Airlie <airlied@xxxxxxxx>
>>>>> Cc: Daniel Vetter <daniel@xxxxxxxx>
>>>>> Cc: Kishon Vijay Abraham I <kishon@xxxxxx>
>>>>> Cc: Vinod Koul <vkoul@xxxxxxxxxx>
>>>>> Cc: Kevin Hilman <khilman@xxxxxxxxxxxx>
>>>>> Cc: Jerome Brunet <jbrunet@xxxxxxxxxxxx>
>>>>> Cc: Martin Blumenstingl <martin.blumenstingl@xxxxxxxxxxxxxx>
>>>>> Cc: Heiko Stuebner <heiko@xxxxxxxxx>
>>>>> Cc: Maxime Ripard <mripard@xxxxxxxxxx>
>>>>> Cc: Guido Günther <agx@xxxxxxxxxxx>
>>>>> Tested-by: Liu Ying <victor.liu@xxxxxxx> # RM67191 DSI panel on i.MX8mq EVK
>>>>> Signed-off-by: Liu Ying <victor.liu@xxxxxxx>
>>>>> ---
>>>>> v1->v2:
>>>>> * Use BITS_PER_BYTE macro. (Andrzej)
>>>>> * Drop dsi argument from ui2bc() in nwl-dsi.c.
>>>>>
>>>>>  drivers/gpu/drm/bridge/nwl-dsi.c                 | 12 +++++-------
>>>>>  drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c    |  3 ++-
>>>>>  drivers/phy/phy-core-mipi-dphy.c                 |  4 ++--
>>>>>  drivers/phy/rockchip/phy-rockchip-inno-dsidphy.c |  3 ++-
>>>>>  include/linux/phy/phy-mipi-dphy.h                |  2 +-
>>>>>  5 files changed, 12 insertions(+), 12 deletions(-)
>>>>>
>>> [...]
>>>
>>>>> diff --git a/drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c b/drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c
>>>>> index cd2332bf0e31..fdbd64c03e12 100644
>>>>> --- a/drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c
>>>>> +++ b/drivers/phy/amlogic/phy-meson-axg-mipi-dphy.c
>>>>> @@ -9,6 +9,7 @@
>>>>>  
>>>>>  #include <linux/bitfield.h>
>>>>>  #include <linux/bitops.h>
>>>>> +#include <linux/bits.h>
>>>>>  #include <linux/clk.h>
>>>>>  #include <linux/delay.h>
>>>>>  #include <linux/io.h>
>>>>> @@ -250,7 +251,7 @@ static int phy_meson_axg_mipi_dphy_power_on(struct phy *phy)
>>>>>  		     (DIV_ROUND_UP(priv->config.clk_zero, temp) << 16) |
>>>>>  		     (DIV_ROUND_UP(priv->config.clk_prepare, temp) << 24));
>>>>>  	regmap_write(priv->regmap, MIPI_DSI_CLK_TIM1,
>>>>> -		     DIV_ROUND_UP(priv->config.clk_pre, temp));
>>>>> +		     DIV_ROUND_UP(priv->config.clk_pre, BITS_PER_BYTE));
>>>>>  
>>>>>  	regmap_write(priv->regmap, MIPI_DSI_HS_TIM,
>>>>>  		     DIV_ROUND_UP(priv->config.hs_exit, temp) |
>>>>
>>>> I'll try to run a test, currently the calculation gives 2, so this would give 1.
>>>
>>> The Amlogic vendor code does:
>>>
>>> /* >8*ui */
>>> #define DPHY_TIME_CLK_PRE(ui)       (10 * ui)
>>
>> This looks like clk_pre time is 10 * ui, which matches the comment
>> '>8*ui' - longer time 8 * ui.
>>
>>>
>>> t_ui = lcd_timing.bit_rate
>>>
>>> t_ui = (1000000 * 100) / (dsi_ui / 1000); /*100*ns */
>>> temp = t_ui * 8; /* lane_byte cycle time */
>>
>> If I read correctly, this temp in vendor code is TxByteClkHS period
>> time in picoseconds. So...
>>
>>>
>>> dphy->clk_pre = ((DPHY_TIME_CLK_PRE(t_ui) + temp - 1) / temp) & 0xff;
>>
>> IIUC, 'dphy->clk_pre' essentially means dphy->clk_pre = DIV_ROUND_UP(10
>> * ui, temp), that is, the time for dphy->clk_pre is no less than 
>> 10 * ui.  The D-PHY spec (v1.2)'s saying is that the minimum time for
>> dphy->clk_pre is 8 * ui.  So, it looks like meson is stricter than the
>> spec.
>>
>> However, the vendor code doesn't seem to match the current meson driver
>> implementation.  The temp in driver code is also TxByteClkHS period
>> time in picoseconds. And, without this patch, I'm assuming that
>> priv->config.clk_pre is 8000.  So, it looks like MIPI_DSI_CLK_TIM1 is
>> set to DIV_ROUND_UP(8000, temp).  This '8000' does _not_ reflect
>> '10 * ui'.
>>
>> So, 3 choices for meson, up to you:
>> 1) If meson really requires the time for clk_pre no less than
>> 10 * ui, then sth like DIV_ROUND_UP(10, BITS_PER_BYTE) can be used.
>> 2) If meson follows the spec's minimum value, then the patch does
>> things right for meson.
>> 3) Force to use 8000 picoseconds for clk_pre, that is, use sth like
>> DIV_ROUND_UP(8000, temp).
>>
>> With this patch applied, do you see any display artifacts by visual
>> check? Or even checked signals through DSI analyzer?
> 
> Thanks for the feedback, I'm currently trying to put the setup back to
> run a test with this change.

I don't see any visible artifacts.

Reviewed-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx> # for phy-meson-axg-mipi-dphy.c
Tested-by: Neil Armstrong <narmstrong@xxxxxxxxxxxx> # for phy-meson-axg-mipi-dphy.c

Thanks,

> 
> Neil
> 
>>
>> Regards,
>> Liu Ying
>>
>>>
>>> PHY Registers only says:
>>> MIPI_DSI_CLK_TIM1	[31:0]
>>> 7:0 	R/W	0	 tCLK_PRE
>>>
>>>> Neil
>>>>
>>>
>>> [...]
>>
>