Re: [PATCH v4 12/12] drm/ast: Implement polling for VGA and SIL164 connectors

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 19 Mar 2024, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> On Tue, 19 Mar 2024, Thomas Zimmermann <tzimmermann@suse.de> wrote:
>> Hi
>>
>> Am 19.03.24 um 10:37 schrieb Maxime Ripard:
>>> Hi,
>>>
>>> On Tue, Mar 19, 2024 at 09:00:32AM +0100, Thomas Zimmermann wrote:
>>>> Implement polling for VGA and SIL164 connectors. Set the flag
>>>> DRM_CONNECTOR_POLL_DISCONNECT for each to detect the removal of the
>>>> monitor cable. Implement struct drm_connector_helper_funcs.detect_ctx
>>>> for each type of connector by testing for EDID data. The code for
>>>> both types of connectors is identical for now. Maybe this can later
>>>> become a common helper function for various drivers.
>>>>
>>>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>>>> ---
>>>>   drivers/gpu/drm/ast/ast_mode.c | 36 ++++++++++++++++++++++++++++++++--
>>>>   1 file changed, 34 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
>>>> index 71cc681d6188f..f740b8706a38b 100644
>>>> --- a/drivers/gpu/drm/ast/ast_mode.c
>>>> +++ b/drivers/gpu/drm/ast/ast_mode.c
>>>> @@ -1344,8 +1344,24 @@ static int ast_crtc_init(struct drm_device *dev)
>>>>    * VGA Connector
>>>>    */
>>>>   
>>>> +static int ast_vga_connector_helper_detect_ctx(struct drm_connector *connector,
>>>> +					       struct drm_modeset_acquire_ctx *ctx,
>>>> +					       bool force)
>>>> +{
>>>> +	enum drm_connector_status status = connector_status_disconnected;
>>>> +	const struct drm_edid *edid;
>>>> +
>>>> +	edid = drm_edid_read(connector);
>>>> +	if (edid)
>>>> +		status = connector_status_connected;
>>>> +	drm_edid_free(edid);
>>>> +
>>>> +	return status;
>>>> +}
>>>> +
>>> Yeah, I think it would be worth turning it into a helper. We have a
>>> number of drivers using some variation of that already
>>> (display-connector, loongson).
>>>
>>> It's probably better to use drm_probe_ddc here too instead of parsing
>>> and updating the EDID property each time we call detect.
>>
>> I see. I'll update the patch accordingly. drm_probe_ddc() doesn't update 
>> the property, which is probably better anyway.
>
> The struct drm_edid based drm_edid_read() and friends do *not* parse the
> EDID (apart from what's necessary to read the EDID) nor update the EDID
> property. You need to call drm_edid_connector_update() separately for
> that. This is by design.
>
> The struct edid based drm_get_edid() does parse and update the property,
> and I think adding that was a mistake that a lot of drivers later
> depended on, and couldn't be removed.
>
> As a design consideration, IMO it's also a fine approach to read and
> cache the EDID and update the property at ->detect, and then use
> drm_edid_connector_add_modes() in ->get_modes to avoid re-reading the
> EDID at that time. It's not uncommon to need stuff from the EDID between
> those hooks.

But if drm_probe_ddc() at ->detect and drm_connector_helper_get_modes()
for ->get_modes are sufficient, nothing wrong with that either.

BR,
Jani.

>
>
> BR,
> Jani.

-- 
Jani Nikula, Intel