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SubjectRe: [PATCH v2 1/8] drm/bridge: use bus flags in bridge timings
On 14.09.2018 02:23, Laurent Pinchart wrote:
> Hi Stefan,
>
> Thankk you for the patch.
>
> On Wednesday, 12 September 2018 21:32:15 EEST Stefan Agner wrote:
>> The DRM bus flags conveys additional information on pixel data on
>> the bus. All currently available bus flags might be of interest for
>> a bridge. In the case at hand a dumb VGA bridge needs a specific
>> data enable polarity (DRM_BUS_FLAG_DE_LOW).
>>
>> Replace the sampling_edge field with input_bus_flags and allow all
>> currently documented bus flags.
>>
>> This changes the perspective from sampling side to the driving
>> side for the currently supported flags. We assume that the sampling
>> edge is always the opposite of the driving edge (hence we need to
>> invert the DRM_BUS_FLAG_PIXDATA_[POS|NEG]EDGE flags). This is an
>> assumption we already make for displays. For all we know it is a
>> safe assumption for bridges too.
>
> Please don't, that will get confusing very quickly. Flag names such as
> DRM_BUS_FLAG_PIXDATA_NEGEDGE don't mention sampling or driving. There's only a
> small comment next to their definition, which will easily be overlooked. I'd
> rather update the definition to cover both sampling and driving, and document
> the input_bus_flags field to explain that all flags refer to sampling.
>

There is history to that, and I'd really rather prefer to keep that similar across the kernel. There is already precedence in the kernel, the display timings (which is a consumer) defines it from the driving perspective too (see DISPLAY_FLAGS_PIXDATA_POSEDGE).

That is why I introduced the bus flags using the same perspective.

If we _really_ want it from sampling side, we should create new defines which are explicit about that, e.g.: DRM_BUS_FLAG_PIXDATA_SAMPLE_NEGEDGE.

But again, since even the display flags use the driving perspective, I'd rather prefer to have it that way also for bridges too.

--
Stefan


>> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> ---
>> drivers/gpu/drm/bridge/dumb-vga-dac.c | 6 +++---
>> include/drm/drm_bridge.h | 11 +++++------
>> 2 files changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/bridge/dumb-vga-dac.c
>> b/drivers/gpu/drm/bridge/dumb-vga-dac.c index 9b706789a341..d5aa0f931ef2
>> 100644
>> --- a/drivers/gpu/drm/bridge/dumb-vga-dac.c
>> +++ b/drivers/gpu/drm/bridge/dumb-vga-dac.c
>> @@ -234,7 +234,7 @@ static int dumb_vga_remove(struct platform_device *pdev)
>> */
>> static const struct drm_bridge_timings default_dac_timings = {
>> /* Timing specifications, datasheet page 7 */
>> - .sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>> + .input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>> .setup_time_ps = 500,
>> .hold_time_ps = 1500,
>> };
>> @@ -245,7 +245,7 @@ static const struct drm_bridge_timings
>> default_dac_timings = { */
>> static const struct drm_bridge_timings ti_ths8134_dac_timings = {
>> /* From timing diagram, datasheet page 9 */
>> - .sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>> + .input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>> /* From datasheet, page 12 */
>> .setup_time_ps = 3000,
>> /* I guess this means latched input */
>> @@ -258,7 +258,7 @@ static const struct drm_bridge_timings
>> ti_ths8134_dac_timings = { */
>> static const struct drm_bridge_timings ti_ths8135_dac_timings = {
>> /* From timing diagram, datasheet page 14 */
>> - .sampling_edge = DRM_BUS_FLAG_PIXDATA_POSEDGE,
>> + .input_bus_flags = DRM_BUS_FLAG_PIXDATA_NEGEDGE,
>> /* From datasheet, page 16 */
>> .setup_time_ps = 2000,
>> .hold_time_ps = 500,
>> diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
>> index bd850747ce54..45e90f4b46c3 100644
>> --- a/include/drm/drm_bridge.h
>> +++ b/include/drm/drm_bridge.h
>> @@ -244,14 +244,13 @@ struct drm_bridge_funcs {
>> */
>> struct drm_bridge_timings {
>> /**
>> - * @sampling_edge:
>> + * @input_bus_flags:
>> *
>> - * Tells whether the bridge samples the digital input signal
>> - * from the display engine on the positive or negative edge of the
>> - * clock, this should reuse the DRM_BUS_FLAG_PIXDATA_[POS|NEG]EDGE
>> - * bitwise flags from the DRM connector (bit 2 and 3 valid).
>> + * Additional settings this bridge requires for the pixel data on
>> + * the input bus (e.g. pixel signal polarity). See also
>> + * &drm_display_info->bus_flags.
>> */
>> - u32 sampling_edge;
>> + u32 input_bus_flags;
>> /**
>> * @setup_time_ps:
>> *

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