[PATCH] drm/bridge: tc358767: Fix odd pixel alignment

[Marek Vasut]

On 11/18/24 4:19 PM, Maxime Ripard wrote:
> On Mon, Oct 28, 2024 at 03:49:42PM +0100, Marek Vasut wrote:
>> On 10/28/24 2:52 PM, Maxime Ripard wrote:
>>> On Mon, Oct 28, 2024 at 01:36:58PM +0100, Marek Vasut wrote:
>>>> On 10/28/24 10:25 AM, Maxime Ripard wrote:
>>>>> On Sat, Oct 26, 2024 at 06:10:01AM +0200, Marek Vasut wrote:
>>>>>> Horizontal Timing Control0 Register 1/2 (HTIM01/HTIM02) Register
>>>>>> bitfields description state "These bits must be multiple of even
>>>>>> pixel". It is not possible to simply align every bitfield to the
>>>>>> nearest even pixel, because that would unalign the line width and
>>>>>> cause visible distortion. Instead, attempt to re-align the timings
>>>>>> such that the hardware requirement is fulfilled without changing
>>>>>> the line width if at all possible.
>>>>>>
>>>>>> Warn the user in case a panel with odd active pixel width or full
>>>>>> line width is used, this is not possible to support with this one
>>>>>> bridge.
>>>>>>
>>>>>> Signed-off-by: Marek Vasut <marex at denx.de>
>>>>>> ---
>>>>>> Cc: Andrzej Hajda <andrzej.hajda at intel.com>
>>>>>> Cc: David Airlie <airlied at gmail.com>
>>>>>> Cc: Jernej Skrabec <jernej.skrabec at gmail.com>
>>>>>> Cc: Jonas Karlman <jonas at kwiboo.se>
>>>>>> Cc: Laurent Pinchart <Laurent.pinchart at ideasonboard.com>
>>>>>> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
>>>>>> Cc: Maxime Ripard <mripard at kernel.org>
>>>>>> Cc: Neil Armstrong <neil.armstrong at linaro.org>
>>>>>> Cc: Robert Foss <rfoss at kernel.org>
>>>>>> Cc: Simona Vetter <simona at ffwll.ch>
>>>>>> Cc: Thomas Zimmermann <tzimmermann at suse.de>
>>>>>> Cc: dri-devel at lists.freedesktop.org
>>>>>> ---
>>>>>>     drivers/gpu/drm/bridge/tc358767.c | 63 +++++++++++++++++++++++++++++--
>>>>>>     1 file changed, 60 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
>>>>>> index 0a6894498267e..7968183510e63 100644
>>>>>> --- a/drivers/gpu/drm/bridge/tc358767.c
>>>>>> +++ b/drivers/gpu/drm/bridge/tc358767.c
>>>>>> @@ -901,6 +901,63 @@ static int tc_set_common_video_mode(struct tc_data *tc,
>>>>>>     	int vsync_len = mode->vsync_end - mode->vsync_start;
>>>>>>     	int ret;
>>>>>> +	/*
>>>>>> +	 * Horizontal Timing Control0 Register 1/2 (HTIM01/HTIM02) Register
>>>>>> +	 * bitfields description state "These bits must be multiple of even
>>>>>> +	 * pixel". It is not possible to simply align every bitfield to the
>>>>>> +	 * nearest even pixel, because that would unalign the line width.
>>>>>> +	 * Instead, attempt to re-align the timings.
>>>>>> +	 */
>>>>>> +
>>>>>> +	/* Panels with odd active pixel count are not supported by the bridge */
>>>>>> +	if (mode->hdisplay & 1)
>>>>>> +		dev_warn(tc->dev, "Panels with odd pixel count per active line are not supported.\n");
>>>>>> +
>>>>>> +	/* HPW is odd */
>>>>>> +	if (hsync_len & 1) {
>>>>>> +		/* Make sure there is some margin left */
>>>>>> +		if (left_margin >= 2) {
>>>>>> +			/* Align HPW up */
>>>>>> +			hsync_len++;
>>>>>> +			left_margin--;
>>>>>> +		} else if (right_margin >= 2) {
>>>>>> +			/* Align HPW up */
>>>>>> +			hsync_len++;
>>>>>> +			right_margin--;
>>>>>> +		} else if (hsync_len > 2) {
>>>>>> +			/* Align HPW down as last-resort option */
>>>>>> +			hsync_len--;
>>>>>> +			left_margin++;
>>>>>> +		} else {
>>>>>> +			dev_warn(tc->dev, "HPW is odd, not enough margins to compensate.\n");
>>>>>> +		}
>>>>>> +	}
>>>>>> +
>>>>>> +	/* HBP is odd (HPW is surely even now) */
>>>>>> +	if (left_margin & 1) {
>>>>>> +		/* Make sure there is some margin left */
>>>>>> +		if (right_margin >= 2) {
>>>>>> +			/* Align HBP up */
>>>>>> +			left_margin++;
>>>>>> +			right_margin--;
>>>>>> +		} else if (hsync_len > 2) {
>>>>>> +			/* HPW is surely even and > 2, which means at least 4 */
>>>>>> +			hsync_len -= 2;
>>>>>> +			/*
>>>>>> +			 * Subtract 2 from sync pulse and distribute it between
>>>>>> +			 * margins. This aligns HBP and keeps HPW aligned.
>>>>>> +			 */
>>>>>> +			left_margin++;
>>>>>> +			right_margin++;
>>>>>> +		} else {
>>>>>> +			dev_warn(tc->dev, "HBP is odd, not enough pixels to compensate.\n");
>>>>>> +		}
>>>>>> +	}
>>>>>> +
>>>>>> +	/* HFP is odd (HBP and HPW is surely even now) */
>>>>>> +	if (right_margin & 1)
>>>>>> +		dev_warn(tc->dev, "HFP is odd, panels with odd pixel count per full line are not supported.\n");
>>>>>> +
>>>>>
>>>>> This should all happen in atomic_check, and reject modes that can't
>>>>> be supported.
>>>
>>>> No, that would reject panels I need to support and which can be
>>>> supported by this bridge.
>>>
>>> Then drop the warnings, either you support it or you don't.
>>
>> These warnings are useful to notify users that something is not right. I
>> find them useful when bringing up systems with this bridge.
> 
> Yeah, because you know what to do about them.
> 
> Unfortunately, you're not the only audience of those messages, and the
> vast majority of the users will have no idea what these warnings are
> about, and no way to fix them.
The net result of removing these warnings will be -- it will be harder 
for me to debug problems with this driver, and the rest of the audience 
will not be able to provide meaningful error reports because the 
warnings will not be present. That does not seem helpful.