linux-kernel - Re: [PATCH v8 08/10] gpu: host1x: mipi: Keep MIPI clock enabled till calibration is done

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Date:   Wed, 5 Aug 2020 17:19:52 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Sowjanya Komatineni <skomatineni@...dia.com>
Cc:     jonathanh@...dia.com, frankc@...dia.com, hverkuil@...all.nl,
        sakari.ailus@....fi, robh+dt@...nel.org, helen.koike@...labora.com,
        gregkh@...uxfoundation.org, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 08/10] gpu: host1x: mipi: Keep MIPI clock enabled till
 calibration is done

05.08.2020 17:05, Dmitry Osipenko пишет:
> 05.08.2020 16:46, Thierry Reding пишет:
>> On Mon, Aug 03, 2020 at 08:42:24AM -0700, Sowjanya Komatineni wrote:
>>> With the split of MIPI calibration into tegra_mipi_calibrate() and
>>> tegra_mipi_wait(), MIPI clock is not kept enabled till the calibration
>>> is done.
>>>
>>> So, this patch skips disabling MIPI clock after triggering start of
>>> calibration and disables it only after waiting for done status from
>>> the calibration logic.
>>>
>>> This patch renames tegra_mipi_calibrate() as tegra_mipi_start_calibration()
>>> and tegra_mipi_wait() as tegra_mipi_finish_calibration() to be inline
>>> with their usage.
>>>
>>> As MIPI clock is left enabled and in case of any failures with CSI input
>>> streaming tegra_mipi_finish_calibration() will not get invoked.
>>> So added new API tegra_mipi_cancel_calibration() which disables MIPI clock
>>> and consumer drivers can call this in such cases.
>>>
>>> Reviewed-by: Dmitry Osipenko <digetx@...il.com>
>>> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
>>> ---
>>>  drivers/gpu/drm/tegra/dsi.c |  4 ++--
>>>  drivers/gpu/host1x/mipi.c   | 19 ++++++++++---------
>>>  include/linux/host1x.h      |  5 +++--
>>>  3 files changed, 15 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/tegra/dsi.c b/drivers/gpu/drm/tegra/dsi.c
>>> index 3820e8d..a7864e9 100644
>>> --- a/drivers/gpu/drm/tegra/dsi.c
>>> +++ b/drivers/gpu/drm/tegra/dsi.c
>>> @@ -694,11 +694,11 @@ static int tegra_dsi_pad_calibrate(struct tegra_dsi *dsi)
>>>  		DSI_PAD_PREEMP_PD(0x03) | DSI_PAD_PREEMP_PU(0x3);
>>>  	tegra_dsi_writel(dsi, value, DSI_PAD_CONTROL_3);
>>>  
>>> -	err = tegra_mipi_calibrate(dsi->mipi);
>>> +	err = tegra_mipi_start_calibration(dsi->mipi);
>>>  	if (err < 0)
>>>  		return err;
>>>  
>>> -	return tegra_mipi_wait(dsi->mipi);
>>> +	return tegra_mipi_finish_calibration(dsi->mipi);
>>>  }
>>>  
>>>  static void tegra_dsi_set_timeout(struct tegra_dsi *dsi, unsigned long bclk,
>>> diff --git a/drivers/gpu/host1x/mipi.c b/drivers/gpu/host1x/mipi.c
>>> index e606464..b15ab6e 100644
>>> --- a/drivers/gpu/host1x/mipi.c
>>> +++ b/drivers/gpu/host1x/mipi.c
>>> @@ -293,17 +293,19 @@ int tegra_mipi_disable(struct tegra_mipi_device *dev)
>>>  }
>>>  EXPORT_SYMBOL(tegra_mipi_disable);
>>>  
>>> -int tegra_mipi_wait(struct tegra_mipi_device *device)
>>> +void tegra_mipi_cancel_calibration(struct tegra_mipi_device *device)
>>> +{
>>> +	clk_disable(device->mipi->clk);
>>
>> Do we need to do anything with the MIPI_CAL_CTRL and MIPI_CAL_STATUS
>> registers here? We don't clear the START bit in the former when the
>> calibration has successfully finished, but I suspect that's because
>> the bit is self-clearing. But I wonder if we still need to clear it
>> upon cancellation to make sure the calibration does indeed stop.
> 
> Apparently there is no way to explicitly stop calibration other than to
> reset MIPI calibration block, but Sowjanya says this is unnecessary.
> 
> Perhaps having a fixed delay before disabling clock could be enough to
> ensure that calibration is stopped before the clock is disabled?
> 

Actually, there is a MIPI_CAL_ACTIVE bit in the status register. Maybe
it needs to be polled until it's unset?