Re: [RFC PATCH 0/4] DSI/DBI and TinyDRM driver

[Emil Velikov <emil.l.velikov@gmail.com>]

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.

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.?
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)?

- 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.
-Emil

[1] https://lists.freedesktop.org/archives/dri-devel/2020-May/266079.html
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