Hi Emil,
Le mar. 16 juin 2020 à 18:47, Emil Velikov <emil.l.velikov@xxxxxxxxx>
a écrit :
Hi all,
Allow me to compare this approach with some work Linus W [1] did a
while back, which I've just noticed.
Pauls' approach:
- Perhaps the shortest one possible
Porting an existing DSI panel to DBI is 3 lines of code - compat line
in the SPI/DSI bridge, a bus_type and
mipi_dsi_maybe_register_tiny_driver() call
The clever use of the DSI type (equal to zero) means that things will
work w/o updating existing dsi hosts and devices in panel/. Yet in the
very unlikely case that the panel does not support DSI, we will still
allow it.
Is there such a case? I assumed that all current DSI device and host
drivers are for DSI panels.
Although thinking about the type I wonder if it can accommodate all
use-cases:
Since we can have a device (panel) capable of DSI+SPI it makes sense
for it to expose the type bitmask, not the host. Although, what if the
host itself supports DSI+SPI.?
Yes, hosts can support more than one type (mine does), so it should
expose a bitmask. As for the panel, even though some can do DSI, SPI
and I8080 DBI, as far as I know the bus used is always set in hardware
(with specific pins set to VCC/GND to select the mode), so this is not
something the host can select. Therefore, the panel driver should
register the mipi_dsi_device with one particular bus type. Note that
the panel driver could very well infer the bus type from the compatible
string.
If the bus type can be changed at runtime (and there's a need for
that), then we would need a bitmask on the panel driver side too, along
with a 'set bus' callback, but I'm not sure it will be required.
Now we can extrapolate that with a host (say fimd/exynos I think)
which supports a SPI panel (s6e63m0) while having
of_graph_get_port_by_id(0)?
I'm not sure I understand, if there is a port #0 in the panel node,
then the tinyDRM driver is not created, and the SPI panel will be
registered with the fimd/exynos host driver. So that should already
work fine.
- Strange (ab)use of the DSI bus for DBI (SPI, 6800, 8080 etc)
We care about existing users (DT) and it sounds unlikely (based on
previous discussion) that DBI + SPI/6800... will make it into DT. So
the current approach seems quite reasonable IMHO.
Linus' approach:
- Clear separation of DSI/SPI
Compat strings are still duplicated, although in separate files
- Minor code motion and slightly more invasive porting overall
Much of the boilerplate can be reduced via helper lib and macros. Even
then it's unlikely we'll reach the 3 line delta as with Paul's
approach.
- Does not handle tiny-dsi (dummy) drm driver
It seems doable, with minor changes
Personally I'm on the fence and a deciding factor for me is if Paul's
approach can handle all the combinations of host/device type support.
That said, the input from people likely to do the work would be highly
appreciated.
Once a decision is made, an illustrative series + todo entry would be
great to have.
Sure. Thanks for the feedback!
Cheers,
-Paul
-Emil
[1]
https://lists.freedesktop.org/archives/dri-devel/2020-May/266079.html
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